Kart seats are pretty important because that's where you sit while you race and being comfortable is an important part of karting.
Generally on a straight kart the seat will be placed in the center of the kart. On an offset kart for oval racing it usually is placed farther left.
There isn't much variety in seats, some different styles and materials that they are made out of and that's about it. Here are the main ones you will see:
Fiberglass seats are popular for the same reasons fiberglass bodies are. They are easy to patch up if they crack a little bit. They also come out with a good finish when painted. It is a strong lightweight material.
Plastic seats are used as well, and have a little more flexibility than fiberglass seats but can look a little less shiny and when broken don't have an easy fix.
If you really want the best out there, there are seats offered made of carbon fiber. This is extremely lightweight and strong and has that carbon fiber look people like.
The seats come in different shapes and sizes but nothing drastically different.
These are laid back so that you are farther out of the air. This benefits in wind resistance and also a lower center of gravity, and less weight transfer.
These are the original seats in karting, and are just more upright than the lay down seats.
There are different sizes of seats and the best way to fit for one is just to sit in them.
To make seats a little more comfortable you can put padding in the seats. Most people just add it on their own but you can buy kits, or find seats that come with pads.
There is more research being done on seats now to make them protect ribs better during karting accidents. The seats are getting better at this, and should get better as more research is done.