"Freedom fighters."
Ah, seriously, vigilante is probably the best word for them. They do seem to have a moral code of behaviour, or more accurately given the lack of centralized leadership and wide nature of the varied populations they draw upon for their membership, myriad moral codes of behaviour that happen to share sufficient points of congruence to permit organized action against any that dare offend those beliefs; outside of those points of congruence, it tends to be difficult to direct them at anything if enough of the individual members may see little point in it. They're also very prideful when taken as a whole; as ace101 noted, offending the group is also a good way to draw their ire. Online, they favor illegal acts to make their displeasure clear, while when acting out in public, they instead tend to prefer peaceful protests, which, to be honest, is simply an anon-specific manifestation of a general trend that treats online affairs as somehow "less real" than the physical world, and hence without consequence. Terrorism, I suppose, isn't necessarily a horrible way to describe it, but it's imperfect in that they don't really seem to seek to inspire terror. Rather, they seem to either want to amuse themselves, or simply punish anyone they think is evil or has the audacity to claim to threaten them, and sometimes, when the stars converge, they can do both at once.