Demigod

Demigods (or demi-gods) are creatures eternal in nature yet capable of ultimate sacrifice. Part god and part other, the demigods of Azeroth wield great power and have occasionally played pivotal roles in the planet's history. Unlike gods, most demigods have never been the objects of worship. Few demigods have temples built in their honor. Priesthoods based on the worship of a demigod are extremely rare. Nevertheless, demigods have been revered in the past, and some of them continue to be respected today.

They are immortal, but they are neither omniscient nor omnipotent. Otherwise, among other things, they would have known immediately of the Burning Legion's first invasion, and they would have been able to thwart it with ease.

Sadly there is no question as to whether or not a demigod can be slain. Several demigods were killed during the War of the Ancients, overwhelmed by demons. In a far more recent tragedy, Cenarius was killed during the Third War by mortals under the influence of demon blood: a group of orcs led by Grom Hellscream.

Ancient Guardians
The Ancient Guardians like Cenarius and Aviana are demigods of Azeroth. The August Celestials are Ancients who remained in Pandaria after the Great Sundering.

The Scourge
The Scourge is a vast army of the undead controlled by the psychic powers of the Lich King (originally Ner'zhul, then Arthas Menethil and now Bolvar Fordragon).
 * Lich King - The Jailer of The Damned and Master of the Scourge.

Other

 * Archimonde - A lord of the Burning Legion. Sargeras had transformed the eredar into monsters and demigods.
 * Azshara - Empress of Nazjatar and ruler of the Naga.
 * Gahz'rilla - worshipped as a deity by the Sandfury trolls in Tanaris.
 * Genesaur - Enormous demigods who date back to the Age of Colossals. They are worshipped by the lesser Primals.
 * Ka'alu - A demigod left in charge of the dread ravens when her consort Anzu vanished into the shadows.
 * Kil'jaeden - A lord of the Burning Legion. Sargeras had transformed the eredar into monsters and demigods.
 * Nhal'athoth - A void revenant whose presence haunts the dark corners of Mac'Aree.
 * Ordos - A yaungol who sacrificed himself to the flame, following the promises of the Fire Lords, in order to become a demigod.
 * Ragnaros - revered by his fiery minions and the dark iron dwarves as a demigod, and lord of the Firelands.

Titans
The titans are demigods who travel across the cosmos bringing order to worlds such as Azeroth. The only titans known are their leaders, a group called the Pantheon:
 * Aggramar - the Avenger.
 * Aman'Thul - the Highfather.
 * Eonar - the Lifebinder.
 * Golganneth - the Thunderer.
 * Khaz'goroth - the Forger.
 * Norgannon - the Lorekeeper.
 * Sargeras - the dark titan.

Former mortals
There are many demigods, but only a few mortals have been called demigods due to the immense power they wield:
 * Xavius - First of the Satyr and the Nightmare Lord.

Notes and trivia

 * The antlers sprouting from Malfurion's forehead did not disgust the dainty night elves. They embraced it as a sign of divinity, as if Malfurion now stood as one of the demigods.
 * In Warcraft: Orcs & Humans and Warcraft II: Tides of Darkness, it is a rank you can achieve.
 * In Warcraft III: Reign of Chaos, Cenarius was a Demigod hero.

Speculation
Demons could be considered demigods. They are immortal just like gods and demigods.

(An interesting note: The term Daemon (a variation of the word demon) is the Latin version of the Greek word "δαίμων", meaning "godlike power fate, god")