O-O : King moves 2 squares to the left, and end up on g1. Rook is flipped to the other side of the King on f1. All squares between the King and Rook must be cleared, with no pieces in between. While moving 2 squares to the left the King can not cross a square which would place him in check.
O-O-O or Queenside castling. King move two squares to the right, and ends up on c1, and the Rook on d1. Same rules apply. King can not move across any squares that would place it in check.
On the Black side...
O-O King on g8, Rook lands on f8.
O-O-O King on c8, Rook lands on d8
Castling is another way of saying: Getting the hell out of Dodge (center of the board).
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