I'm going to divy Warlockry up into three posts: The basics (damage and crowd control), the situational (pets and curses), and development (talents and equipment). We are a sophisticated class, after all, and there is so much to manage.
You say you like to torture your enemies to death with the slow dissolution of their bodies and summon demons with the crystalized remains of their pathetic souls? Excellent, then the way of the warlock may be for you.
Warlocks excel at two major tasks: first eliminating opponents with a little mana and a fair amount of patience and second keeping enemies occupied while you do it.
Most of the warlock finest damage spells do damage over time (DoT). Curse of Agony (more on curses later), Corruption and Immolate are the basics. The channeled drain life and drain soul spells, as well as the channeled Area of Effect (AoE) Rain of fire and Hellfire all mete out pain over time. Even seed of corruption turns [an] opponent[s] into [a] ticking timebomb[s] (beware the angry mobs!) with a DoT wick. Affliction, which specializes in DoTs adds two to the mix: siphon life, a fire-and-forget trickle of health from your victim(s) to you, and unstable affliction, which makes anyone foolish enough to dispel your DoTs very sad.
One consequence of this DoT mania is that warlocks take their time, even with cakewalk mobs. It takes time to load enemies up with our best DoTs, and the other DPS may given our victims a merciful end before a warlock has really even started playing with them. On the other hand, warlocks who keep their DoTs up on a long lasting opponent and live to tell the tale will rank high on the damage meters.
Warlocks have four crowd control options and can use three of them simultaneously: Fear is first. It sends just about any non-boss enemy running all over the place. It's one of the best crowd controls in the game. You can chain it, you can hit them with dots without losing the fear and a couple of points in a first tier affliction talent make it hard to resist. So you can kill them without them ever getting close to you. On the downside, in crowded environments, it easily has the potential to assemble all your opponents to come eat you together. The brief terror attached to death coil and the brief AoE horror of howl of terror are similar. But they have short durations with long cool downs. The short duration reduces the linking risk, but the long cool downs limit the crowd control value.
The second is banish. It holds a single demon or elemental in stasis. It cannot do anything, nothing can be done to it, and it stays still. It also cannot be broken early and has to break before it can be recast. DoTs lose time, but do no damage, and cannot be reapplied until banish expires.
The third and fourth relate to demons. A warlock's succubus can seduce humanoids. This spell takes 1.5 seconds to cast, and requires the succubus's undivided attention to maintain (humanoids are so fickle). When it does break, whether early or not, it takes another 1.5 seconds of target freedom to recast. Unlike polymorph, it doesn't heal the target, and it cannot be recast before it expires. But it can be broken by damage to the target, or any interruption to the succubus. Additionally, since it is the succubus casting, not the warlock, I don't believe it gains the penetration and spell hit of the warlock.
The fourth is to enlave a demon. The enslaved demon won't be as effective or as loyal as one you summon yourself. But it takes an enemy and makes a (feisty) pet. Unfortunately, your normal pet will head off for other parts when you do this, meaning no simultaneous seduction and enslavement. This is crowd control of last resort.
Now, your victims are seduced, banished, enslaved, playing pattycake with your demon, or running screaming with their hair on fire as their bodies unravel. You can knit, pick flowers, plan your next meal, compose poetry, or just relax. We warlocks are a leisurely class. But if you are in a hurry, you can accelerate the process. Shadow bolts are slow casting, but do pack a punch. They are the main non-DoT damage tool of the warlock. Other tools include shadowburn (requires a soul shard, short cooldown), death coil (long cool down), soulfire (long cast time, requires a soul shard), searing pain (distracts victims from your pet or tank), and incinerate (better with immolate up, but still not amazing from what I've seen).
Destruction warlocks also get conflagrate, which consumes an immolate in a burst of damage (best to cast just when the victim thinks it's almost over) and shadowfury, a quickcasting, short cool down AoE damage and stun. Fun to use on casters and I'm told it's great for detonating Seed of Corruption.
Ah, Seed of Corruption, that reminds of our Area of Effect (AoE) spells. Whenever faced with a great many victims there's no way to strike fear into them more quickly than AoEs. Rain of Fire allows you to select an area in which any hapless victims will be subject balls of fire dropping from above while you concentrate. Lovely, but not always sufficient. Hellfire channels something more deadly, but also dangerous to you. You see, you burn along with your targets, and the fire originates from you. Don't worry though, your party members are safe. Then comes my personal favorite, seed of corruption. This puts a DoT on your victim, and when they are damaged up to a certain level, they explode with shadowy goodness, sharing it with everyone around them, and keeping none for themselves. Sadly, one must choose between this and the usual corruption, but it is a small price to pay. The damage can be truly impressive and will often make you quite popular (with the victims).
So that summarizes the basic tools in a Warlock's toolbox. Next up, the situational tools: pets and curses. Tune in later for more silly exposition on warlocks.