More updated version of the answer above
World of Warcraft
When you first get World of Warcraft it's completely free until your character reaches level 20 (There's also a gold limit in this period).
- Then you can buy World of Warcraft, In that you gain the level 1 - 60 levelling experience and the level 1 - 60 dungeons and raids as well as the
- Paladin, Warrior, Hunter, Druid, Mage, Warlock, Shaman, Rogue and Priest classes and the
- Human, Dwarf, Night Elf and Gnome Alliance races and the
- Orc, Troll, Forsaken and Tauren Horde races as well as access to the
- Mining, Herbalism, Skinning, Alchemy, Enchanting, Blacksmithing, Engineering, Leather working, and Tailoring primary professions and the
- Cooking, Fishing and First Aid secondary professions
In addition to the original World of Warcraft product Blizzard has now given away the Burning Crusade expansion free with the original game so you also get access to the
- New Draenei Alliance race plus the
- New Blood Elf Horde race and the
- New Outland 60 - 70 levelling experience plus
- New dungeons and raids as well as
- Flying mounts and
- A new Primary Profession 'Jewelcrafting'
Wrath of the Lich King Expansion
In this expansion you gain access to the
- New Death Knight class and the
- New Northend 70 - 80 levelling experience plus
- New dungeons and raids as well as
- A new Primary Profession 'Inscription'
Cataclysm Expansion
In this expansion you gain access to the
- New Worgen Alliance race plus the
- New Goblin Horde race and the
- New 80 - 85 levelling experience plus
- New dungeons and raids as well as
- A new Secondary Profession 'Archaeology'
Further Update 2014: Starter edition and Vanilla WOW now contains The Burning Crusade, Wrath of the Lich King and Cataclysm. The Mists of Pandaria and upcoming Warlords of Dreanor are still paid for separately. The starter edition is also limited to characters of level 20 or less. You can't level past that unless you buy vanilla (or "classic") WoW.
