Warlocks are your traditional pet caster class with some utility thrown in. The pet mainly allows a Warlock to solo, since the classes primary damage spells come from DOT (damage over time) spells. The Warlocks most potent spells require soulshards that have to be harvested in combat. The Warlock community has long complained about the inability to get shards in PVP has severely limited their effectiveness in end game as well as over all server numbers. That being said Warlocks are far from weak they have a utility that is only rivaled by possibly the shaman. Through the use of Soulstones or healthstones they create they can resurrect, heal , teleport people to themselves (requires help of 2 other people).
Warlock is one of only 2 classes that get a free mount at 40.