Exoplanets are planets that do not revolt around the sun but around other stars.
There are 4 main categories of exoplanets:
Gas giant: Planets like Jupiter that are made out of gas
Neptunian: Planets whose size is close to that of Neptune
Super-Earth: Planets that are similar to earth but have a much greater size
Earth-like or terrestrial planets: Planets that are similar to earth
The way to detect exoplanets is by exploring the gravitational effects they have on the start they are revolving around.