There are many ways people cheat, but the biggest thing in my opinion is ratio cheating. This means they spoofed there ratio to report something higher then it actually is. They do this in many different ways using some type of ratio cheating software or a modified client. However, people who cheat will always get caught and trackers usually run scripts to search for people using this type of technique.
Here is a great tutorial by JustJenna on how to spot a cheater. u should check it out cause its very helpful:
http://www.torrent-invites.com/tutor...o-cheater.html