Hero class

A hero class is a character class derived from the base classes, but with exclusive perks. Examples of these perks include a higher starting level, better starting gear, unique abilities, and class mounts. Another example of a perk is that your abilities can run off something other than mana, rage or energy. The only current hero class, the death knight, uses the rune system which no other class has. Prerequisites are required to "unlock" the hero class for play. The concept of a hero class is derived from the Warcraft III heroes.

Blizzard's Meaning of "Hero Class"
When Blizzard said "Hero Class" they never had any intention of saying a class is more powerful as if you were a hero. It just means a class is more unique in play style. The word hero often seems to confuse players to think the opposite of what Blizzard defined it as.

"'It's also important to keep in mind that even though the death knight is a hero class, that doesn't mean it is more powerful than the other classes, just that it will offer a very different playing experience.'"
 * — Quoted from the official Q&A on Death Knights.

Death Knight
At Blizzcon 2007, Blizzard announced that the death knight hero class was to be a new addition in the Wrath of the Lich King expansion pack. Nethaera posted on the Wrath of the Lich King forums that the current plans are to introduce one new hero class per expansion set. Death knights do not have a mana, rage, or energy bar; instead they have a runic power bar and use runes.

The death knight can have two roles: tanking and damage dealing (or DPS). They do not use shields while tanking, instead using two-handed weapons or dual-wielding one-handed weapons. Non-melee magic abilities can also play a role during combat.

Prerequisites
To create a death knight, the player must already have at least one level 55 (or higher) character and the Wrath of the Lich King expansion pack. Only one death knight can be created per realm. Death knight creation is limited to realms that have been open for 90 days or for players who have a level 55 character on the realm.

Unique perks

 * Death knights start at level 55.
 * Death knights have no race restrictions.
 * Death knights quickly obtain a full set of gear right in their starting area.
 * Because they start the game at such a high level, they are given the flight paths for the original continents.
 * Their Runeforging ability allows death knights to enchant their own weapons.
 * Only death knights can obtain the Acherus Deathcharger and Winged Steed of the Ebon Blade mounts.

Bard
The Bard hero class was announced on April 1st 2008, as a Blizzard April Fools' Day hoax.

According to the announcement, the Bard "can vanquish evil with the power of ROCK!" and wields a two-handed "axe" (that is, an electric guitar). Screenshots revealed a new "Fret Bar" interface (resembling that of the Guitar Hero games) powering the Bard's "Groove" resource system. Blizzard claimed that the Wrath of the Lich King expansion would ship with a Frostmourne-themed "guitar-based keyboard peripheral".

Two Talent trees were revealed, including Metal Talents (such as Starstruck, Epic Solo, and Axe Specialization), and Punk Rock Talents (such as Shred, Mosh Pit, and Rock 'N Roll Racing).

Original announcement: http://www.worldofwarcraft.com/wrath/features/bard/bardclass.xml

Pre-World of Warcraft hero classes
Paladins were a hero class in Warcraft III: Reign of Chaos but were changed to a normal class when World of Warcraft was released. Paladins were also originally considered being made into a hero class in WoW.

Other possible hero classes
In Blizzcast 7, it was revealed that while making Wrath of the Lich King, Blizzard considered a number of ideas for the hero class, which they narrowed down to four front runners. Besides the death knight, they also considered:
 * Necromancer
 * Runemaster
 * Monk

Although they did not ultimately choose to implement the class, some of the ideas for the necromancer class were incorporated into the death knight instead.

Note that, during the Blizzcast interview, J. Allen Brack mentions a figure of "29 hero classes that we've got completed". However, some sources quoting the interview frame this as "Brack joked that ..." , and the interviewer who was face to face with him during the interview laughed after he made the statement. However, he delivers the statement deadpan, and some people have taken the comment seriously and literally. In any case, regardless of the actual number, it is clear that Blizzard has a number of ideas beyond the one we've seen implemented so far.