A shill, plant, or stooge according to Wikipedia is a person who publicly helps a person or organization without disclosing that he has a close relationship with that person or organization.
In Internet slang, a troll is someone who posts inflammatory,extraneous, or off-topic messages in an online community, such as an online discussion forum, chat room, or blog, with the primary intent of provoking readers into an emotional response or of otherwise disrupting normal on-topic discussion. (Wikipedia)
The difference being that the troll while annoying and disruptive generally does not have an agenda but the Shill does.
The Shill wants to annoy and disrupt but above all he wants to misinform and destroy the ability of people to inform other people in order for an agenda to be implemented. This agenda can be to destroy unions, opposition to politics, opposition to another big supermarket or competition in business or politics.
The use of Shills is common and should surprise no one. It is not paranoid to accept the premiss that if you upset powerful people they will endeavour to make the annoyance go away and maligning is an excellent way of doing so.
I don’t often call people shills. In fact for those of you who have read my blog for a long time will have noticed that I have only in the last two months on occasion pointed out someone I thought showed the hallmarks of shill behaviour.
Here is a link to my new page with 25 shill behaviour patterns you might want to use to recognise and explain behaviour in people you get in contact with through political or other activism. No need to get paranoid but if you are an activist chances are you will be challenged or engaged by a person acting as a shill for the people you are active against.
To practice here is a link to the page of the person I called a shill yesterday and both his post and answers to the comments he got to that post is an excellent training exercise. For the fun of it try to count how many patterns he shows.