Q:

What's difference between graha and loka?

As per hinduism there are nine graha (planets) and Earth is not included in this list. Here are following nine graha

Surya (Sun)
Chandra (Moon)
Mangala (Mars)
Budha (Mercury)
Guru (Jupiter), the Guru (Master/Teacher) of the Devas(Gods)
Shukra (Venus), the Guru of the Asura(demons)
Shani (Saturn)
Raahu (North Node/Ascending Node)
Ketu (South Node/Descending Node)

Question is, all graha and earth looks same. If we see images of planets and Earth (taken by ISRO, NASA etc) then they looks same. Shape of them are same. So
it's clear that appearance is not the reason of difference between Earth (loka) and graha (planets). So What's difference between graha and loka? what's the reason Earth is considered as loka, not as graha?

A:

Graha

The Sanskrit word "Graha" is often taken as the equivalent of the
  English word planet. This is a very simplistic translation which hides
  the deeper meaning of this word. "Graha" refers to an entity that has
  the power to "seize, lay hold of, or grasp". Therefore, Graha means
  “to grasp” meaning the planets possess, or take hold of us. This
  meaning is intimately related to the meaning of the nakshatras (also
  called lunar mansions, or a 27-fold division of the zodiac), which are
  described in similar terms. Graha therefore goes beyond the word
  planet and describes the ability of celestial events to either
  influence or predict occurrences on earth.

Loka

Loka is a Sanskrit term meaning "world" or "a particular division of
  the universe." The most frequent division of the universe in Hinduism
  is the tri-loka: Earth, heaven and atmosphere. Some sources consider
  there to be 14 worlds. These are the seven higher worlds, found above
  the Earth; and the seven lower worlds, found beneath the Earth.

Now the 14 Lokas include seven higher worlds and seven lower. The earth is considered to be the lowest among the seven higher worlds. These 7 higher worlds are the 7 Vyahrtis and 7 Patalas.
The 7 Vyarthis

Satya-loka:Satya-loka is also known as the Brahma loka and this loka is the topmost loka in the material universe. This is the abode of truth where atmans are released from the need of rebirth.
Tapa-loka: This is the abode of tapas or of other Gods and Goddesses.
Jana-loka: This is the house of the sons of Lord Brahma.
Mahar-loka: This is the dwelling of great sages and laissez-faire beings like Markendeya and other rishis.
Svar-loka: This expanse is between the sun and the polar star. This is the heaven of Lord Indra. Indra, other Gods, Rishis, Gandharvas and Apsaras live here. This place is full of heavenly ecstasy where all the 330 million Hindu Gods (Devatas) live in along with the king of Gods, Indra.
Bhuvar-loka or Pitri Loka: This is the place of the Sun, planets, stars and the space between earth and the Sun. It is an actual region, the atmosphere, the place of soul and essence.
Bhur-loka:This is the place where the Earth dwells. According to the Vishnu Purana the earth is simply one of thousands of billions of occupied worlds like itself located in the Universe.

The 7 Patalas

Atala-loka: This loka is ruled by Bala – son of Maya – who enjoys supernatural powers. By one yawn, Bala created three types of women – Svairinis – the self willed women who marry men from their own group; Kaminis – lustfull women who marry men from any group; and finally Pumscalis – those women who gave themselves fully and continue changing their partner.
Vitala-loka: This Loka is ruled by the God Hara-Bhava – an incarnation of Lord Shiva. The residents of this dominion are decorated with gold from this region.
Sutala-loka: Sutala is the kingdom of the virtuous demon king Mahabali.
Talatala-loka: This loka is the kingdom of the demon-architect Maya, who is well-versed in necromancy.
Mahatala-loka: Mahātala is the abode of many-hooded Nagas – the sons of Kadru – lead by Krodhavasha band of Kuhaka, Taksshaka, Kaliya and Sushena.
Rasatala-loka: This is the home of the Danavas and Daityas, who are the everlasting foes of Devatas.
Patala-loka: The lowest territory of the materialistic universe is called Patala or Nagaloka. Vasuki is the rular of this Naga region. Here live quite a lot of Nagas whose hoods are decorated with a jewel and thus enlighten this realm with the lights reflected from the jewels.

If we consider the scientific approach, as you can read here and here The Earth is not a planet for the same reasons that Pluto is no longer considered as one. Strange but True: Earth Is Not Round
Reference 1 and 2

