The art of Trollin' is to call out stupid people and say "Hey, stupid people! I see what you're doing, and I don't like it"
Trollin' can be done anywhere, though it's mostly done online. And is the simple yet effective task of annoying the fuck out of someone, untill they give up or get mad. Usually resulting in the person leaving the forum/chat room.
It is often speculated as to what Trolls get out of Trollin'. Though most Trolls will tell you that it brings then gleeful joy to successfully Troll a stoopid.
To avoid being Troll'd, simply do not feed the Troll.
For example Google "Example of Trollin'"
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