What is the goal of Sicilian defense?

Generally to achieve active piece play with black. Each type of Sicilian has different plans and ideas.

very "simple".
Basic principle is to take centre with pawn.
Black uses a wing pawn to control centre.
So black aims to take advantage of the centre : in open sicilian, 1.e4 c5 2.Nf3 d6 (or other) 3.d4 c5xd4 4.Nxd4, black has more pawns than white in centre (e-d vs e).
White compensates this with active play (knight on d4 seems strong), but then chess is concrete and all this is not enough... This is just the beginning and it is very abstract, depending on black and white set up.

In the broadest possible terms, black's goal is to force the trade of a wing pawn for a center pawn, so he has an extra center pawn, and then use the resulting half-open c-file as an avenue for his rooks.
Black is willing to give up a lead in development and a kingside space advantage in order to do this.