A large group of cowards sitting behind their computers being assholes, on the internet, just because they can get away with it through their anonymity and supposed proxies. They usually come from 4chan while other members of anonymous always somehow roots to 4chan even if they came from another website.

Anonymous has two purposes, to create trouble (trolling), and to masturbate furiously (fapping) emptying out their children as a part of their daily routine.

They create trouble so that they may laugh at it (calling it lulz). Anonymous will laugh at anything and anyone that man has ever thought of or seen. At the end of the day, they will laugh, masturbate, and sleep.

If one is targeted by anonymous they must treat them the same as trolls are treated. Don't react, don't respond, they'll lose interest and they'll eventually move on.

As a more broad term:
Anonymous is people without a face or identity. Anyone can be anonymous, as long as they choose not to show their face and keep their identity private.

In a way Anonymous could be said to represent the general masses of today's society that is unbound to the burden of one's own identity.

Formerly known as the "final boss of the Internet", Anon has become simply a bunch of weeaboo faggots who wish to be a part of something.

Anonymous talks to hear itself talk. Only anonymous finds anonymous funny.

Anonymous mocks blacks, as its own tiny penis makes it feel inadequate.
Anonymous gay bashes, for inside it craves the butt-loving.
Anonymous hates furries, because calling attention to them draws attention away from its own twisted perversions.
Anonymous is a pedophile.
Anonymous has Oedipal desires, but can only dream and fap to them.
Anonymous actually faps to everything it mocks.
Anonymous primarily consists of 14 year old boys, or at least all anonymous is at heart.

If you confront an Anonymous with its true name, it will explode.
Anonymous knows no fear... because it's easy to be brave when your target can't fight back. Like beating up a kid in a wheelchair.

Anonymous will downvote this definition so it can feel better about itself, because that will somehow give it "lulz", which it exists entirely upon.

United as one, divided by zero. We are Anonymous, We are legion, We do not forgive. We used to be actually cool, and worth the laugh and the reminder not to take things too seriously. But in our haste to remind others of their canny ability to ignore reality, we've expended all this energy, more energy than they do avoiding reality. So now we're kindof a sad mess of dumbasses still clinging to an outdated manifesto that may as well be the instructions on someone's VCR manual. We're sad. Sad sad people. We eat Doritos for breakfast, lunch and dinner. We write minimal annoying scripts that do fuckall except annoy people for 12 seconds. We're quick to brag about our endeavors because what press we've recieved has given us a false sense of validity. But don't fool yourselves. We're a bunch of sad, undersexed, testosterone-filled highschool dropouts who work fast food, live off our parents and smoke too much weed. Move along now. We've hit the snooze button on our 15 minutes of fame. Our voices are silent.

a bunch of fags who think that posting things on message boards actually does something. They claim to be "one" , yet they are unorganized aside from a few friends still living with their parents organizing a spam brigade

Bunch of mc donalds employees who have seen v for vendetta one too many times. Some of them do it for the lulz which is acceptable, but some of them actually think they're some sort of revolutionary movement - lame.

Self proclaimed "Hackers on Steroids". Bunch of 14 year olds who probably never have been socially accepted at school. Usually making agressive comments towards African-Americans. Not exactly a powerful force due to the fact if a situation occurs in which their community (4chan) shuts down , they will whine like sissy bitches. Secondly , no one IRL knows about them , let alone cares lol. But hey , it's for the lulz right?

Anonymous is the ultimate internet tough guy. An OL psycho bully, butthurt about being everybody's bitch and being pwnd IRL all the time, set himself to win the only way he kind of knew how, which is also ironical since it was his's smart odds that usually got him raep to begin with. Being the IRL pussy that he is, decided to hide his actions' persona behind true anonymity; in an undifferentiated gang of other sorry ass losers. Those actions more often that not, resulted in major lols, as expected of such fail IRL lolcows. Finally getting tired of being the laughing stock of the universe in all imaginable possible ways, they decided to better justified it and say that it was on purpose and hence forged a new inducive adage, that they 'did it for the lulz