The fork technic concists in threatening 2 value pieces usualy by 2 diagonals.

So than in the next move, one of these both should be captured.  

The castle is a kind of construction with gold and silver general to protect the king.

Depending on the oppenent strategy (static rook, ranging rook) you build one or another castle.

The openings are the first moves and basement of a game.

They usualy reflect the player strategy that will be used (ranging rook / static rook).