A cheat square is a black square that is added to a puzzle that doesn't increase the word count. For example, a black square in any corner is a cheater. It "cheats", I guess, because it shortens the word needed for that space and makes it easier to fill. Generally, fewer black squares are desirable, but certainly cheaters are occasionally used and published.
As to your other question, I'm not sure what you're asking. If ":" was provided as a clue, the answer might be "colon" or something. I've also occasionally seen "smilies" used as clues; these often contain the :.
