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.
Anonymous faps to porn, hates on everything, faps to porn, hates on everything, faps to porn, hates on everything, faps to porn, time to sleep.
Despite all this, one can never be completely Anonymous on the internet even with proxies.
"There is no way to hide on the internet, no matter how hard you cover your tracks you can get caught. You're not invincible." -Matthew George (Anonymous)
Anonymous could be considered geniuses.
Anonymous could be considered retards.
Anonymous is sometimes even brilliant.
To learn more about Anonymous please visit /b/ (google b) to create your own opinion on the matter. It's not that bad of a place. Go browse with an open mind. Don't mind the gay porn, gay furries, cartoon ponies, dicks, gore, white supremacy, misogynist beliefs, men wearing woman's clothing and exposing their penises (called traps), pictures and moving pictures (.gif) of dead people, moving pictures of people dying, FBI luring with child pornography, young boy pornography, infinitely re-posted content etc etc and just see what anonymous is with your own eyes.