I'll quote myself from a small Warlock-guide I wrote some time ago:
2. Specs
There are many ways to spec a Warlock. All the below builds are tested by me. Every build has it's pro and cons and you should feel free to spend points differently than I did. Currently I'm sticking to DS/ruin for personal reasons, NOT because it's the best build available.
SM/ruin or 30/0/21This is the go-to cookie cutter build for a Warlock. It features a high damage output both in PvE and PvP and solid solo farming capabilities. It provides an Imp for the tanks, high DoT damage for multiple targets and good direct damage on bosses.
MD/ruin or 0/30/21A spec usually used by horde Warlocks because of the improved aggro handling via MD and Imp. A MD/ruin Warlock can decide to either use his Imp for the imp. BP and 30% aggro reduction or to sacrifice his Succubus for an additional 15% Shadow damage.
DS/ruin or 7/21/23Similar to MD/ruin, only that this spec sacrifices the ability to use Imp+MD for PvE and Succubus+MD for PvP for better dps in PvE (imp. Corruption, imp. LT).
Full Destruction or 7/9/35Often considered a PvP build it doesn't do too bad in PvE either, especially in AQ40/Naxx, where mobs don't have fire immunity and Mages use imp. Scorch. It features high burst damage and improves direct damage casts with a short cast time (SP and Immolate). As a plus it brings an improved Imp without downsides.
NF/Conflagrate or 18/1/32Similar to Full Desctrution, it only sacrifices a bit of SP crit and Demonology survivability for even more damage. Along with SM/ruin THE glass cannon build for a Warlock.
SM/DS or 30/21/0The best farming build. It combines SM and DS for a massive +25% Shadowdmg. It has all the Affliction and Demonology utility but lacks direct damage talents. It's great for farming and for PvE encounters where you can deal most of your damage via DoTs.
SL/Sburn or 7/33/11While SM/ruin is a PvE build that can also be used for PvP, SL/Sburn is a PvP build that can also be used for PvE. It features SL, MD and improved Demons for maximum survivability and Shadow Burn for additional burst damage. Upon reaching the twin emperors in AQ40, one Warlock should spec this build to make the fight easier until the encounter is on farm status.
NF/SL or 20/31/0The beast spec for PvP. SL, DS and DL make killing this Warlock extremely difficult and NF provides the lock with the much needed burst damage, although it's a bit RNG. Counting gear, talents and life drained this Warlock can take far more than 10k damage punishment and still live. It is the worst imaginable spec for PvE though.