General Army Notes:
The Chaos army is a fairly powerful army, but its lack of missile fire (with the exception of the Spawn) means that it must close with an opposing army and assault as soon as possible. Combined assaults is the key here - Chaos Warriors coupled with Marauders in an assault gives a much better chance of winning than just a lone Chaos Warrior unit.
Foot Units:
Chaos Warriors: They are 150 points per unit, and one unit is mandatory. Even with the high points cost, this is a tough unit that will stick around and do some damage.
Marauders: Marauders have basic human stats with a decent save. 60 points per unit, one unit is mandatory. Marauders make a good, reliable core group of units.
Ogres: Ogres have to charge humans on initiative, which makes them rather unreliable at times (this can be exploited by a very crafty opponent). For 45 points less than a Chaos Warrior unit, you get a unit almost as tough, but you can only have one unit.
Trolls: Troll units are 110 points, and you can only have three units, but they are strong in an assault. Trolls take a -1 to orders rolls, making them hard to control at times. Their regeneration ability allows them to stick around long enough to get into an assault and do some damage.
Cavalry/Chariots:
Marauder Cavalry: Same stats as foot Marauders. Cheap cavalry at only 90 points, but you are limited to only 4 units. Cavalry are always a good investment, and the Marauder Cavalry are no exception.
Chaos Knights: Same stats as foot Chaos Warriors. 200 points pet unit, max of four units in the army. Chaos Knights will tear through almost any unit in an assault, but be careful with them as they can get cut off from supporting units.
Chaos Hounds: While their stats may not seem like much, Chaos hounds are also cavalry and only cost 30 points pet unit. You can have two units. Chaos hounds are a worthy investment for a Chaos army, both as screening units and as cheap units to aid in an assault.
Chariots: Marauder Chariots with basic Marauder stats. Chariot units are effective in aiding an assault, but will most likely not win any battles on their own.
Monsters:
Harpies: Harpies may seem good on the surface, but with their wide bases and no character they are rather limited in their effectiveness. As in WHFB, Harpies make an alright harassment unit, but don't try to make a straight-on assault with them.
Dragon Ogres: Even with their high cost, Dragon Ogres can be a good buy. Their power in an assault can be more crushing than even Chaos Knights. They should still be supported by other units, however, or they will quickly get swamped by the enemy and destroyed.
Chaos Spawn: Even with only one base per unit, they are an excellent buy for Chaos, as they are the only unit in a Chaos army with a ranged attack! Their survivability means they will stick around longer than most units. You can take two units, but they must be in a brigade with more units. Spawn are brainless, so they can't move if they aren't joined with other, more numerous units. 110 points may seem like a high price to pay for one unit, but they can add a bit of tactical flexibility to your army that may be otherwise lacking.
Characters:
General: Definitely worth the points.
Hero: The Hero's main strength is in his ability to lend extra strength to a unit in an assault.
Sorceror: For only 10 points more than a normal Hero, you gain the ability to cast spells.
Chaos Dragon Mount: The Chaos Dragon has a breath attack with a 20cm range which can be useful for opportunity fire. A 100cm fly move will let your character join any unit he can join, which can be extremely useful in bolstering a part of your line that is giving way in an assault. Chaos Dragons are the only monster mounts a Chaos army can take.
Special thanks to Dalkwave for his help on this tactical overview!