Beastmen
Where the Warriors are Chaos tanks and Daemons are Chaos cheese, the Beastmen are the true Chaos horde. The Beastmen call themselves the children of chaos, descended from outcast mutants hurled into the cursed woods of the Drakwald. They are nuisance to the human empire and the Bretonnians, but only the Wood Elves know how truly dangerous they are. The Beastmen's favourite activities include toppling waystones, erecting herdstones, kidnapping/raping/eating humans and just plain old smashing shit! They absolutely hate anything built or anything that is pure and make it a priority to destroy and defile that which the civilized races cherish. In short, they're a bunch of drunken anarchist fucktards that hang out in the woods surfacing only to pillage and defile - which is kinda awesome if you're into that sort of thing!
History
The Beastmen weren't always "the Beastmen" as we know them today. For the longest time, they were actually part of a unified chaos army, but due to the fact that people seemed to like to roll with either all beast, mortal or daemon army lists, Games Workshop in their infinite wisdom decided to gradually split off the armies.
Beasts in Hordes of Chaos
Squishier, quicker units than mortal warriors and good for setting up charges, which daemons couldn't really do. A couple of special and rare units that were "beastly," but nothing much.
Beasts of Chaos
They were pretty bad in this edition being essentially the bitch of chaos and with almost NO redeeming qualities of their own. When they were first split from the Hordes of Chaos some years ago, they were nothing more than an unruly (literally) band of meat shields for your warriors and daemons. In fact, because they could move faster, it was preferable to field a few beast packs to soak up arrow fire and act as distractions while fast-moving daemons got the flanks and warriors hammered the front ranks.
They had core units that mixed low toughness, lightly armoured Ungors with spears and tough, brawny brawlers in the form of Gors. They were also skirmishers that could essentially scout and would lineup with the Gors in the front and the Ungors jabbing from behind with their spears. The alternative was to supplement with warhounds or take a bunch of strong, slow bestigors, but they were too expensive and almost entirely useless.
The only interesting thing about the Beasts was that they had access to a lot of cool special and rare choices, like Chaos trolls and Ogres, Dragon Ogres and Shaggoths, etc. However, Hordes of Chaos had better shit and later on, the Warriors of Chaos would absorb almost all of the Beasts' specials and rares!
Beastmen
Sometimes referred to as "the FAIR chaos army," it more or less resembles a normal army in Warhammer Fantasy Battle with cheap, expendable troops, average leaderships and no immunity to psychology (with the exception of a few units). The things they otherwise don't have are cavalry and missile unit blocks, but they make up for it with other cool shit.
The Beastmen today are a solid stand-alone army featuring decent core choices that can ambush, a rape-tastic special choice and a kick-ass rare slot buffet!