Photorealism
Long before there was any form of digital art, artists would create art by getting a photograph and gridding it to recreate the image square by square. Photorealism is a term for making an image using paint that looks like it could have been taken straight from a camera if not, look better.
Using this in games is rather challenging, this is because as you are seeing the game every little thing you can see needs to be rendered in real time. So due to this it is quite processor heavy.
Cel Shading
Cel shading or maybe more known as Toon shading, is opposite to Photorealism. This type of art is used to get a non-photorealistic, this is used to make 3D computer graphics to make an image look flat by using less shading colour.
This style is used to make your artwork look like a cartoon character or setting as if it was out of a comic book. The name 'Cel Shading' (short for celluloid).
Abstraction
Abstraction is known for its imagery that isn't representable, focusing on shape, colour and movement. Little or perhaps no storyline.
Exaggeration
This is popular style for all types of public entertainment as it helps grab the viewers attention. It is also a very effective style in games as it allows the gamers to become immersed into an alternate reality.