yeap, dota is a custom map in warcraft 3, if you've ever heard of the custom map 3 corridors, its similar to that but better. You control one hero and progressively get stronger getting items etc. that in mind, the game is like alterac valley (from wow), where 2 sides battle against eachother in hopes of killing the commander (in dota its a castle u destroy), you can grab it from
www.getdota.com , if you've got any questions you can pm me and i'll help you learn the basics