bittorrent is a protocol, a bit like http or ftp. It provides a way of sharing large files at high speed.
yes, it will work on a mac, but not sure what client you would use.
with all of these .torrent sites you need to share big style or you get banned. I leave my sharing pc on 24/7 to keep good ratios. If you're not happy about leaving a computer online then you might have problems....