One thing to add to what Bylk said, they became very popular in what's called "Deep Web" or "Invisible Web". Deep web is an area of the internet where everything is available and open to anybody. It's a completely different "internet" than what we use and interact with on a daily basis. Just getting access to it takes a toll on the average user due to the fact that it's very secure and well hidden.
Just to give you a hint or an idea of how big "deep web" is...
There is everything in the deep web, people hire assassins, sell weapons, hire spies. I have actually seen all of those with my own eyes on one of the forums in deep web.. i was shocked. It's such a big world that they have their own currency, called the Bitcoin and 1 Bitcoin = 10 US dollars.
Anyways, this group "Anonymous" gained so much popularity in that part of the internet that it started recruiting the best of the best (hackers) to fight for the openness and freedom of the internet.