depends on who you ask. to a lot of people the "shill" is the person that does not agree with them. A few people will now come on here and tell you that I am the shill, lol. You will never really know i fear.
Basically, to me, the shill is the person that only focuses on derailing threads with pointless comments. We can be talking right now about shills, and real people will come on with their theories, but the shill will come and post something like "wtf would you know you useless twat. we all hate you". It is ap ost that has nothing to do with the conversation. It is designed to make everyone reading it mistrust the person they are talking to, and if anyone bites and takes the bait, you end up with a big fight, then a dead thread. Hope that helps.