He wears a vest over a t-shirt with shorts and is quite fat.
He is not as tough as he seems as in many episodes he has a secret enjoyment with cooking.
Nelson is in Bart's class and in the early episodes of The Simpsons he was Bart's Bully and in later episodes he and Bart became friends.
Nelson Muntz: Ha ha.
*Lisa Puts a hat on and walks off*
*Old lady falls into a bin*
Nelson: *Points* Ha ha!
*A very tall man drives by in a small car*
Nelson: *Points* HA HA!
Tall man: Do you find something comical about my appearance while I was driving my automobile?
Tall man: Everyone needs to drive a vehicle, even the very tall. This was the largest auto that I could afford. Should I therefore be made the subject of fun?
Nelson: I guess so.
Tall man: Whatdja, would you like it if I made fun of your misfortune. HUH? Maybe we should find out!
*Tall man pulls Nelson's pants down*
Tall man: Now MARCH!