It's an RTS/RPG hybrid. A standard top down view strategy map with two opposing bases, with tiered fortifications. Each base is already generated at the beginning of a match and automatically creates computer controlled units and sends them to battle. The human player is a single hero trying to affect the outcome of that battle.
10 players make up two teams (5v5). They each begin the game as weak lvl 1 heroes. Initially the focus is on killing the computer controlled units (creeps) to gain experience and gold for items. As the hero gains levels and items they are able to do greater amounts of damage to the opposing creeps and opposing human controlled heroes. At high level heroes will destroy entire creep armies and siege base defenses. The game is won by destroying a primary building at the base of the opposing side.
What makes it fun/compelling to me.
Games are short with no character permanence. No standard RPG style grinding. Just a 30-45min RTS style chessmatch
Minimal wrote build orders. Unlike the first 10mins of most RTS matches
Controlling a single unit lessens the clickfest aspect of most RTS style games
Coordinated teamplay is vital, but this can also result in new players getting flamed mercilessly
Beginning with a full base with defenses and creeps by default prevents the standard RTS formula of rush > boom > turtle > rush. Opens up more options for gameplay and strategic depth. Dota is designed in such a way that games tend to explode rather than stalemate. Hero HP is additive, but damage is multiplicative (avoiding games that drag on despite being over).