Self-proclaimed as "this century's Dennis the Mennis" (last century), Bart was pretty much the focus of almost every episode for the first few seasons. Since then, Bart has evolved beyond his stereotypical catchphrases like "Aye Carumba!" & "Don't Have A Cow Man!" into a sort of troublemaker with a conscience.
Lovable Homer doesn't have a care in the world, or a living brain cell left in his head. Nevertheless, Homer's incompetance continues to make us laugh and cry, sometimes hysterically. Homer's character has replaced Bart as the centerpoint of the Simpsons franchise.
Poor Marge does her best to keep her family healthy and funtionable. Best of luck to her, but i don't think that'll happen until the final episode (God, what a horrible thought!). Always looking on the bright side of things, Marge is still learning how to survive in the real world outside of her home. Aren't we all?
Lisa is the black sheep of the family. Just a tad bit more intelligent and cultured than the rest of the Simpson Clan. Although she will occasionally engage in their crazy antics, she is more often than not the voice of reason amoung the group. Her favorite hobby is playing the saxaphone, quite badly I might add.
Maggie is the silent, pacifier-sucking, Mr. Burns-shooting baby of the Simpson family. Although she doesn't say much, she seems to be, ironically, the most chronically intelligent member of the family.