Dispel can be used to describe a number of spells that deal with removal of spells and effects;
Dispel (Priest, Felhunter and Doomguard) Removes one to two harmful effects from a friendly target, or one to two beneficial effects from a hostile target. Doomguard uses rank 1.
Cleanse (Paladin) Removes one or two harmful effects from a friendly target.
Purge (Shaman) The opposite of Cleanse, removes one to two buffs from a hostile target.
Whether one or two effects are removed depends on the rank of the spell used.
Doomguard only has access to rank 1 of Dispel, and can only remove one effect per cast.
It is the same for the felhunter, except for the cooldown, and that he heals himself when using Devour. Keep in mind that while enemies cannot see what buffs you have on (unless an enemy mage happened to have

cast detect magic on you), they can see what buffs you cast during a fight, and may dispel you when the fight starts, to ensure that any buffs are removed from you.
As a sidenote, buffs that are gained by potions can be dispelled, except for Elixers and Flasks, which are the most powerful.
Spells
A great amount of our spells are magical, and can be dispelled;
Unstable Affliction
Corruption
Immolate
Siphon Life
Drain Life
Drain Mana
Demon Armor
The Spellstone effect
Shadow Ward
Detect Invisibility
Unending Breath
Pet Spells
Imp’s Fire Shield
Voidwalker’s Sacrifice
Felhunter’s Tainted Blood
Talents
Soul Link can be dispelled, but dispel must be cast on the pet.
Nightfall