Planet
A planet is a spherical celestial body orbiting a star. They occur in a great variety of different compositions, masses and surface conditions. Planets with diameters below 100 kilometers are usually classified as planetoids or asteroids.
The final appearance of a planet depends on its formation process, which means where and under what circumstances it formed. Planets may accumulate gigantic atmospheres, exceeding their original size and mass, or they may rest without any atmosphere at all. With a sufficient mass, a planet is able to accumulate so much gas, that it starts to undergo nuclear fusion, turning into a protostar. In this state it begins to emit energy, as its core temperature and density increase. Theoretically, all supergiants of the classes S and T may be regarded as "failed stars". Planets can also be changed to a Class M environment by terraforming.
The United Federation of Planets has numerous different classifications for planets depending on their mass, composition, surface environment and atmospheric compounds.
Lists
A List of canon planets in the Star Trek universe can be found at Memory Alpha's List of Planets. Planets unique to the UFoP:SB 118 Star Trek universe can be found here, at 118Wiki's List of Planets. Additionally, several lists for specific groups of planets exist: