user254: Having read descriptions of perception and consciousness (two of the five skandhas), I still don't understand the difference.
Is there an important difference? What is the difference and why is it important?

user295: 
Consciousness - knows what is felt and results in pleasant, unpleasant or neutral sensation as a reflexive mechanism. Say in the past you had a bitter experience with X and you see him the 1st reaction would be unpleasant sensation. This is a reflex or knee jerk reaction even before you fully recognise the person. See: Viññāṇa (Consciousness)
Perception - Analyses what has some to your consciousness further and names, rebers and recognises the object. Now you recognise and name the person, this is Mr. X, he has these looks, he was a bully at school, etc. or even perhaps this is another person who looks like Mr X. See: Saññā (Perception)


user-1: The definitions given in the texts are not so clear. Here's a translation for a sutta defining sañña and viññāṇa:

“And why, bhikkhus, do you call it sañña? ‘It perceives,’ bhikkhus, therefore it is called sañña. And what does it perceive? It perceives blue, it perceives yellow, it perceives red, it perceives white. ‘It perceives,’ bhikkhus, therefore it is called sañña.
[...]
“And why, bhikkhus, do you call it viññāṇa? ‘It cognizes, ’ bhikkhus, therefore it is called viññāṇa. And what does it cognize? It cognizes sour, it cognizes bitter, it cognizes pungent, it cognizes sweet, it cognizes sharp, it cognizes mild, it cognizes salty, it cognizes bland. ‘It cognizes,’ bhikkhus, therefore it is called viññāṇa.
-- SN 22.79

These alone are not very helpful, though. This is how the Visuddhimagga puts it:

For though the state of knowing (janana-bhava) is equally present in perception (sanna) , in consciousness (vinnana) and in understanding (panna), nevertheless, perception is only the mere perceiving of an object as, say, 'blue' or 'yellow'; it cannot bring about the penetration of its characteristics as impermanent, painful and not-self. Consciousness knows the object as blue or yellow and it brings about the penetrating of its characteristics, but it cannot bring about, by endeavouring, the manifestation of the [supramundane] path. 

On the other hand, in her analysis, Sue Hamilton1 presents saññakhandha as, roughly, the aspect of the mind that recognizes, discriminates, conceptualizes and identifies (e.g. naming) something. Conversely, viññāṇakhandha is the awareness of or consciousness of. Furthermore, we know from the suttas that viññāṇakhandha is six-fold (paired to the 6 senses) and it is one of three things necessary for contact (the others being the presence of a functioning organ and a corresponding object). We also know from the suttas that contact seems to be defined as the first event that triggers a sequence of mental activities. Thus, viññāṇakhandha seems to refer to the bare awareness of "something", or more generally, to the sequential awareness of each stimulus brought by the senses that the mind inclines to, or becomes aware of (manifesting contact). From then on, vedana follows, along with a proliferation of mental activities (including more sophisticated recognition activities performed by sañña).

1: Identity and Experience: The Constitution of the Human Being According to Early Buddhism

user6994: In relation to the five aggregates, the consciousness aggregate solely refers to main minds, such as the eye-consciousness, the ear-consciousness, and so forth. This excludes mental factors. Mental factors are consciousnesses but they are not included within the category of consciousness aggregate. Except feelings and perception (or 'discrimination', etc.) mental factors are included in the category of compositional factors.
Because perception is a consciousness, it is clear and knowing. It means that it has the capacity of taking the aspect of the object it apprehends, and it knows/apprehends its object.
According to Mahayana Abhidharma, perception is one of the five omnipresent mental factors. It means that perception accompanies every single main mind. Here is an example: an eye-consciousness seeing blue takes the aspect of blue, and that aspect is apprehended by the perception factor that is in the retinue of the eye-consciousness as well. A main mind and its concomitant mental factors share five similarities that are explained by Yeshe Gyaltsen in his Necklace for Those of Clear Awareness Clearly Revealing the Modes of Minds and Mental Factors.
Regarding the entity of perception, the Compendium of Knowledge says:

QUESTION: What is the defining characteristic of discrimination
  [Tib. 'du shes, literally, aggregation-knowing]?
RESPONSE: It has the characteristic of knowing upon aggregation. It has the entity of
  apprehending the sign and apprehending the mark, through which one
  designates an expression to objects of perceptions, hearing,
  differentiation and knowledge.

Yeshe Gyaltsen explains further:

Just as it has been said above, it is a knower that, upon the
  aggregation of the three – object, sense power, and primary
  consciousness – apprehends the uncommon sign of an object. Also, A
  Discussion of the Five Aggregates says:

Later in the text, he explains two division:


The apprehension of a sign with regard to an object means to
  apprehend it through individually distinguishing the object's
  particularities, such as blue, yellow, and so forth.
The
  apprehension of a sign with regard to a convention means to apprehend
  it through individually distinguishing the convention's
  particularities, as when one thinks, "This is a man; that is a woman."



254: <p>Having read descriptions of perception and consciousness (two of the five skandhas), I still don't understand the difference.</p>

<p>Is there an important difference? What is the difference and why is it important?</p>
 295: <ul>
<li>Consciousness - knows what is felt and results in pleasant, unpleasant or neutral sensation as a reflexive mechanism. Say in the past you had a bitter experience with X and you see him the 1st reaction would be unpleasant sensation. This is a reflex or knee jerk reaction even before you fully recognise the person. See: <a href="http://www.themindingcentre.org/dharmafarer/wp-content/uploads/2009/12/17.8a-Khandha-5-Vinnana-piya.pdf" rel="nofollow noreferrer">Viññāṇa (Consciousness)</a></li>
<li>Perception - Analyses what has some to your consciousness further and names, rebers and recognises the object. Now you recognise and name the person, this is Mr. X, he has these looks, he was a bully at school, etc. or even perhaps this is another person who looks like Mr X. See: <a href="http://www.themindingcentre.org/dharmafarer/wp-content/uploads/2009/12/17.4-Khandha-3-Sanna-piya.pdf" rel="nofollow noreferrer">Saññā (Perception)</a></li>
</ul>
 -1: <p>The definitions given in the texts are not so clear. Here's a translation for a sutta defining <code>sañña</code> and <code>viññāṇa</code>:</p>

<blockquote>
  <p>“And why, bhikkhus, do you call it sañña? ‘It perceives,’ bhikkhus, therefore it is called sañña. And what does it perceive? It perceives blue, it perceives yellow, it perceives red, it perceives white. ‘It perceives,’ bhikkhus, therefore it is called sañña.</p>
  
  <p>[...]</p>
  
  <p>“And why, bhikkhus, do you call it viññāṇa? ‘It cognizes, ’ bhikkhus, therefore it is called viññāṇa. And what does it cognize? It cognizes sour, it cognizes bitter, it cognizes pungent, it cognizes sweet, it cognizes sharp, it cognizes mild, it cognizes salty, it cognizes bland. ‘It cognizes,’ bhikkhus, therefore it is called viññāṇa.</p>
  
  <p>-- <a href="https://suttacentral.net/en/sn22.79" rel="nofollow">SN 22.79</a></p>
</blockquote>

<p>These alone are not very helpful, though. This is how the Visuddhimagga puts it:</p>

<blockquote>
  <p>For though the state of knowing (janana-bhava) is equally present in perception (sanna) , in consciousness (vinnana) and in understanding (panna), nevertheless, perception is only the mere perceiving of an object as, say, 'blue' or 'yellow'; it cannot bring about the penetration of its characteristics as impermanent, painful and not-self. Consciousness knows the object as blue or yellow and it brings about the penetrating of its characteristics, but it cannot bring about, by endeavouring, the manifestation of the [supramundane] path. </p>
</blockquote>

<p>On the other hand, in her analysis, Sue Hamilton<sup>1</sup> presents <code>saññakhandha</code> as, roughly, the aspect of the mind that recognizes, discriminates, conceptualizes and identifies (e.g. naming) something. Conversely, <code>viññāṇakhandha</code> is the <code>awareness of</code> or <code>consciousness of</code>. Furthermore, we know from the suttas that <code>viññāṇakhandha</code> is six-fold (paired to the 6 senses) and it is one of three things necessary for <code>contact</code> (the others being the presence of a functioning organ and a corresponding object). We also know from the suttas that <code>contact</code> seems to be defined as the first event that triggers a sequence of mental activities. Thus, <code>viññāṇakhandha</code> seems to refer to the bare awareness of "something", or more generally, to the sequential awareness of each stimulus brought by the senses that the mind inclines to, or becomes aware of (manifesting <code>contact</code>). From then on, vedana follows, along with a proliferation of mental activities (including more sophisticated recognition activities performed by <code>sañña</code>).</p>

<hr>

<p><sup>1</sup>: <em>Identity and Experience: The Constitution of the Human Being According to Early Buddhism</em></p>
 6994: <p>In relation to the five aggregates, the consciousness aggregate <strong>solely</strong> refers to main minds, such as the eye-consciousness, the ear-consciousness, and so forth. This excludes mental factors. Mental factors are consciousnesses but they are not included within the category of consciousness aggregate. Except feelings and perception (or 'discrimination', etc.) mental factors are included in the category of compositional factors.</p>

<p>Because perception is a consciousness, it is clear and knowing. It means that it has the capacity of taking the aspect of the object it apprehends, and it knows/apprehends its object.</p>

<p>According to Mahayana Abhidharma, perception is one of the five omnipresent mental factors. It means that perception accompanies every single main mind. Here is an example: an eye-consciousness seeing blue takes the aspect of blue, and that aspect is apprehended by the perception factor that is in the retinue of the eye-consciousness as well. A main mind and its concomitant mental factors share five similarities that are explained by Yeshe Gyaltsen in his <em>Necklace for Those of Clear Awareness Clearly Revealing the Modes of Minds and Mental Factors</em>.</p>

<p>Regarding the entity of perception, the <em>Compendium of Knowledge</em> says:</p>

<blockquote>
  <p>QUESTION: What is the defining characteristic of discrimination
  [Tib. 'du shes, literally, aggregation-knowing]?</p>
  
  <p>RESPONSE: It has the characteristic of knowing upon aggregation. It has the entity of
  apprehending the sign and apprehending the mark, through which one
  designates an expression to objects of perceptions, hearing,
  differentiation and knowledge.</p>
</blockquote>

<p>Yeshe Gyaltsen explains further:</p>

<blockquote>
  <p>Just as it has been said above, it is a knower that, upon the
  aggregation of the three – object, sense power, and primary
  consciousness – apprehends the uncommon sign of an object. Also, A
  Discussion of the Five Aggregates says:</p>
</blockquote>

<p>Later in the text, he explains two division:</p>

<blockquote>
  <ol>
  <li>The apprehension of a sign with regard to an object means to
  apprehend it through individually distinguishing the object's
  particularities, such as blue, yellow, and so forth.</li>
  <li>The
  apprehension of a sign with regard to a convention means to apprehend
  it through individually distinguishing the convention's
  particularities, as when one thinks, "This is a man; that is a woman."</li>
  </ol>
</blockquote>
