cel based animation.

How is it animated?

Firstly, all characters and backgrounds are drawn onto paper (labeled from 1 - 25, due to there being 25 frames in a second of film), then, the characters and backgrounds are traced onto an individual cel which will be flipped onto the reverse side and painted in. The cel is then flipped back onto the shiny side and take two shots from your camera, repeat this process until you've finished the scene (be warned, cel takes at least six months to produce the actual animation).

This video can explain it more clearly.

Examples:

Regular Show:

Regular Show is about the fantastical misadventures of two park-keepers (nee' slackers) named Mordecai (a blue jay) and Rigby (a raccoon). 

Dexter's Laboratory:

Dexter's Laboratory is all about how one boy genius, Who will defend his secret, hidden, laboratory from his "stoopid" sister, DeeDee.

Powerpuff Girls:

Powerpuff girls is about three super powed girls (Blossom, Bubbles and Buttercup) saving the city of Townsville from the clutches of evil.