Sicilian Defence

The Sicilian Defence is the most popular response from black to the King pawn opening.
It starts with the moves
1. e2-e4 c7-c5

The main idea of this opening is to control the d4 centre square with the c-pawn and try to retain both the e-pawn and d-pawn for stronger centre control.
There are several variations in this defence, most of which lead to sharp tactical play. White usually tries to attack on the king side while Black goes for a queen side attack.

Details on Wiki, ChessGames, ChessPoster, TheChessWebsite, Chess & Caissa
Blog on Sicilian & Downloadable PDF on Introduction to Sicilian