user17427: Thero, Bhadanta, and Venerable.. are words for addressing the old monks, abbots in pagodas and/or temples. I really do not know how they are ranked in Buddhist Sangha. Which title is the highest... and what else?  

user471: From AccessToInsight's Glossary of Pali and Buddhist Terms:

samaṇera (samaṇerī):
  Literally, a small samaṇa; a novice monk
  (nun) who observes ten precepts and who is a candidate for admission
  to the order of bhikkhus (bhikkhunīs).
bhikkhu:
  A Buddhist monk; a man who has given up the householder's
  life to live a life of heightened virtue (see sīla) in accordance with
  the Vinaya in general, and the Pātimokkha rules in particular. 
thera:
  "Elder." An honorific title automatically conferred upon a
  bhikkhu of at least ten years' standing.
mahāthera:
  "Great elder." An honorific title automatically
  conferred upon a bhikkhu of at least twenty years' standing.

The female equivalents are samaṇerī, bhikkhuni, theri and mahātheri.

17427: <p><strong>Thero, Bhadanta, and Venerable..</strong> are words for addressing the old monks, abbots in pagodas and/or temples. I really do not know how they are ranked in Buddhist Sangha. Which title is the highest... and what else?  </p>
 471: <p>From AccessToInsight's <a href="https://www.accesstoinsight.org/glossary.html" rel="nofollow noreferrer">Glossary of Pali and Buddhist Terms</a>:</p>

<blockquote>
  <p>samaṇera (samaṇerī):<br>
  Literally, a small samaṇa; a novice monk
  (nun) who observes ten precepts and who is a candidate for admission
  to the order of bhikkhus (bhikkhunīs).</p>
  
  <p>bhikkhu:<br>
  A Buddhist monk; a man who has given up the householder's
  life to live a life of heightened virtue (see sīla) in accordance with
  the Vinaya in general, and the Pātimokkha rules in particular. </p>
  
  <p>thera:<br>
  "Elder." An honorific title automatically conferred upon a
  bhikkhu of at least ten years' standing.</p>
  
  <p>mahāthera:<br>
  "Great elder." An honorific title automatically
  conferred upon a bhikkhu of at least twenty years' standing.</p>
</blockquote>

<p>The female equivalents are samaṇerī, bhikkhuni, theri and mahātheri.</p>
