Depending on your point of view, they are internet terrorists or defenders of freedom. They are pro-free speech and will what they can, including illegal activity, to prevent censorship. Unfortunately, they also pull some massive dick moves based on their anonymity. They often embody the worst qualities of humanity, including lack of empathy, prejudice including racism and sexism, and are quick to anger and vengeance. Oftentimes they will engage in activities that are viewed very positively by the internet community at large while engaging in activities that earn nothing but scorn. For what it's worth, most of what they do is in jest (i.e. for the lulz), but they have a pretty twisted sense of humour. They are the Lokis of the 'Net.
Again, whether or not they are a positive or negative force depends on your point of view.