What does iTunes count as a "skip"?
Let's start with the obvious.

A song is considered played once the reproduction ends. It does not matter if you've listened to all of it or just skipped to the last second of the song, once the song automatically changes to the next one (or ends the reproduction list), the play count is incremented.

Now, the less obvious: 

If a song is skipped in it's first 10 seconds, the skip count is incremented. If the song is skipped when the are 10 seconds remaining for the song to end, the play count is incremented. If you skip the song in between, nor the skip nor the play count is incremented.

The skip can triggered by either using the left arrow key, pressing the fast forward key or just selecting a different song a hitting enter.