crossword construction

[xmenot58]

I have seen published crosswords having answers with no clue but instead this symbol -: Can somebody tell me what this means? Also, what exactly is a cheat square?

[SJS]

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 :.

[jorkel]

I don't think the '-' has any consistent meaning from puzzle to puzzle.  What it may signify is that the crossword involves some trick which defies the normal convention, and the clue for that portion of the puzzle is subsumed within some other entity of the puzzle -- such as a neighboring entry.
-Joe Krozel