Priests heal. They get some other, incidental stuff, but mostly they heal.
Mages nuke. They summon food and water, and they create portals to major cities, but mostly they nuke.
Warlocks are a pet class. They start with an imp, who's weak and mouthy but a decent nuker. Later they get a voidwalker, a big dark blue blob that tanks for them. Then comes the succubus, a sexy little number with crowd control, a whip and an attitude. Felhunter keeps bad spells off the warlock and does fair damage. Other than that, warlocks DoT and nuke and hand out health stones and rez stones (on a timer, got to watch that) and heal the pet and sacrifice the pet when things go rough.
Warlocks are my favorite of the three, mainly because they're complicated to play. Warlocks and druids are busy players.
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