While a classic in its own right and the great-granddaddy of all ARTS/MOBA titles, DotA isn't actually the most popular game of its kind on the market today. That honor goes to its descendant and primary rival, League of Legends.
To understand where League came from, however, a bit of crash course in DotA's history is required.
Defense of the Ancients started out as a mod for the popular Real Time Strategy game Warcraft III: Reign of Chaos. Its original form, simply titled Defense of the Ancients, was developed by a modder by the username of "Eul." The mod exploded in popularity and was picked up and tinkered with by other modders, most significantly by Steve "Guinsoo" Feak, who created the mod's most famous iteration, Defense of the Ancients: Allstars, from which today's DotA 2 is a direct continuation.
Guinsoo would pass Allstars down to its current developer, an anonymous individual named "Icefrog," who works at Valve today balancing DotA 2. Guinsoo partnered with Brandon Beck and Marc Merrill, the founders of Riot Games, to create a spiritual successor to Defense of the Ancients, one that would run on its own engine instead of relying on the assets of a third party.
On launch, League of Legends was in direct competition with another spiritual successor to the original DotA mod, titled Heroes of Newerth. Due to its more distinct play style and visual charm, League edged out HoN, which is now a virtual ghost town due to DotA 2 doing everything it set out to do, but better.
League players and DotA players have a longstanding feud over which game is "better." Objectively speaking, DotA 2 is massively more complex and has a steeper learning curve, which is a possible reason DotA 2 players decry League as being dumbed-down and boring and League players say DotA 2 is unintuitive and confusing.
In terms of maps, League's Summoner's Rift is much smaller and more dense than the Valley of the Ancients in Dota 2. Also, map boundaries and lanes are defined in League not by destructible areas of trees (of which there are none in League), but by formations of stone which can be hopped over or dashed through at certain narrow points.
In all, players of both consider League to be more fun to play, and DotA 2 to be more fun to spectate. If you swear off one and worship the other, you'll never know. They're both free!
