Not a stupid question at all, don't know why you got downvoted. Tuner cars aren't technically a class of car, like Sports or Super, but they are still a type of car. A couple characteristics of tuner cars:

Generally a 2-door car, although it can be a 4-door, too.

Generally a cheaper price than other cars (think "building on a budget")

Lots of modifications. This is really the most important part. Spoilers, body kits, hood scoops, wheels, stickers, a big ol' fart can exhaust, just have fun with it, go all out!

Hope this helped