What is the Colle-Zukertort?
The Colle-Zukertort is a chess opening system where White locks up the center, causing transportation issues for Black, and can then attack the enemy King.
A typical position is shown below.
Of course, Black might put his pieces elsewhere, but most of the time he has a lot of pieces all "stuck" on the Queen-side and no real way of attacking the White King directly. Note how Black does not have an easy way of opening the center to counter-attack. His pieces just do not have many good places to go!
White, on the other hand, can throw several pieces into a King-side attack. His King is so safe that he can also often afford to send some pawns up the board as well. Black has to walk a fine line between defending his King and attacking on the Queen-side, and often he falls off that fine line to a roaring lava-flow of pain!
Even when Black uses his moves purely for defense, he is not assured safety:
You can also read an article I wrote about the C-Z at chessville.