Defense of the Ancients has its roots in the Starcraft scenario Aeon of Strife, where users controlled extremely strong units (of their choice) amidst a massive Protoss battle.
With the release of Warcraft 3, many users found the extreme micro and power of the "Heroes" clashed with the traditional goals of a Real Time Strategy. In an effort to reconcile these, many map makers divorced the roles of Heroes from traditional RTS. The map maker Eul, in particular, copied the model of AoS where the RTS elements were controlled by the AI and Heroes were controlled by the players.
Eul's map (called Defense of the Ancients) was met with a lot of success however, after the release of Warcraft 3's expansion The Frozen Throne, the map maker did not update it. Because of its popularity a large number of "clone" DotA maps arose to leverage the new expansion.
The most popular of these "clones" was a map by Steve "Guinsoo" Freak called DotA Allstars. Today, Freak's legacy is considered the true DotA map, and people refer to it by the name DotA. Freak added key features, such as: a recipe system, a neutral boss AI, and references to Eul.
Initially games of DotA were organized via channels and clans began to forum. A member of one of these clans, Steve "Pendragon" Mescon, eventually went on to create a website known as dota-allstars.com. It was latter dubbed "official" by Freak.
Towards the end of his association with the map, Feak primarily worked on optimizing the map before handing over control to another developer after version 6.01. The new author, IceFrog, added new features, heroes, and fixes. Each release is accompanied with a changelog. IceFrog was at one time highly reclusive, refusing to give interviews; the only evidence of his authorship was the map maker's email account on the official website and the name branded on the game's loading screen.
Freak has since moved on to full time game development (Riot Game's League of Legends), while IceFrog continues to develop the game (as well as work on Value and S2's remakes of the game). The game still remains popular but has given away to more recent additions to the genre.