Leech

Leeching is when your client downloads but doesn't allow downloading.
It is important to notice that it becomes a leech when it doesn't allow. This means that if you download from someone but you are not uploading to him at that very moment you aren't leeching as long as you are allow him to download at some moment (when that client gets to the top of the queue).
Such clients are banned as soon as they are detected by other clients and they are called leechers or bad guys.

It is also important to notice that the difference between a ban and a leech is that banning doesn't allow uploading, but doesn't allow downloading either from that client.