OMG, warlocks are teh r00l. I'm a bit worried about the eldritch bolt ability though, but I haven't read the abilities word for word yet, just speed-read 'em.
As for how the warlock is different than the sorcerer...
"Born of a supernatural bloodline, a warlock seeks to master the perilous magic that suffuses his soul. Unlike sorcerers or wizards, who approach arane magic through the medium of spells, a warlock invokes powerful magic through nothing more than an effort of will."
In other words, warlocks don't cast spells; they have spell-like abilities. This is importaint because it means they can't apply metamagic feats to their invocations (though they can use feats like Quicken Spell-Like Ability and the such).
What I haven't seen mentioned yet is the fact that they gain damage resistance (x/cold iron, 1/cold iron starting at level 3 and going up to 5/cold iron at level 19) and they eventually get a short-duration fast healing (1, 2 and 4 at levels 8, 13 and 18, respectively).
WIthout taking a feat to give them more Invocations (there's one right here in Complete Arcane that lets them do just that), they max out at 12 tricks at level 20.
They have a very darkish tone to them. Definitely give off a "bad guy" vibe.
Eldritch Blast, the power I'm kinda worried about, is apparently useable at will and, at level 20, deals 9d6 damage. As a ranged touch attack. Some of the invocations modify this blast (such as Eldritch Cone, which turns it into a cone attack).