CCG can be an abbreviation for Collectible Card Game or Customizable Card Game, depending on which game it is (or, more correctly, which manufacturer it is).

An example of a Collectible Card Game is Babylon 5 by Precedence.
The masters of Customizable Card Games are undoubtedly Decipher, with Star Wars CCG, Star Trek CCG and others.

An interesting thing to note is that Magic: The Gathering (and every other card game by Wizards of the Coast for that matter) is not a CCG -- it's actually a TCG; a Trading Card Game.