Defense of the Ancients is actually based upon the "Aeon of Strife" custom map for the original StarCraft, but was brought to Warcraft III as DotA. Many different developers worked on the map, but IceFrog--now working on Dota 2 with Valve--has updated the game since 2005 with DotA-Allstars.

The gameplay of DotA or a DotA-like game is straightforward with two teams of hero units attempting to destroy a key structure in the opposing team's base. Multiple pathways to the enemy base, called lanes, are defended by turrets, which must be destroyed before it is safe to progress.

Each team is assisted by computer controlled units that spawn at regular intervals and mindlessly traverse each lane before meeting up with the other teams "creep" units. Left alone, the creeps will generally cancel each other out and the frontline will stay in the middle of the two bases along a lane. Players will need to assist in killing creeps to push the frontlines toward the enemy turret and eventually destroy it.

Your hero unit, chosen at the beginning of the game, has unique spells and abilities, which can be leveled up as experience is gained. Killing enemy heroes will grant large experience bonuses and the team with the most hero kills will usually win. Players can also kill neutral creeps in areas around the lanes for extra experience, but are vulnerable to enemy attack while doing so.

Teamplay is extremely important as one unskilled player can throw a game out of balance by constantly dying, effectively "feeding" the enemy team experience. It is not an easy game to jump into and the communities of the various DotA titles are notoriously abrasive. If you are feeling up to the task, however, there are a few ways to jump into a DotA or DotA-like experience right now.

Defense of the Ancients - WarCraft III custom map (PC, Mac)
Probably the hardest to get into at this point. Its out-of-date feature set and hardcore community will make it tough for new players to jump in. If you want to brave these dangerous waters, you'll need a copy of Warcraft III: The Frozen Throne, the latest DotA map, and a lot of patience.

Heroes of Newerth (PC, Mac, Linux)
This is essentially a clone of DotA, but with a lot of enhanced features added by S2 games like matchmaking, stat tracking, and more. The community is just as difficult to new players, but it will be a bit easier to get into than the WC3 DotA. It will run you $30 for an account, but trials are available through current players. Find out if one of your friends has any trial invites to spare!

League of Legends (PC)
One of the original developers of DotA and its community manager worked on League of Legends with Riot Games. The game is free-to-play and is supported by microtransactions, making it a great title to start with. It also has some nice tutorials, AI opponents, and a 3v3 mode, which is a bit more manageable for new players. LoL is a very approachable game.

Monday Night Combat (Xbox 360)
While Monday Night Combat isn't a clone of DotA, it does use some of the mechanics of the sub-genre in a third-person shooter game. This is where shooter fans may want to start as it will teach you the concepts creeps and pushing lanes in a fun shooter.