But first, I'll try and explain it. You have a map of a large forest which you view from a top-down perspective familiar to any RTS player. There is a base at the top right, and another at the top left, and three "lanes" run between them, dotted with towers that belong to one side or the other. At regular intervals, a back of soldiers - "creeps" - spawn in your base and run towards the enemy base, mirrored by enemy creeps. Naturally they meet in the middle and normally, they just cancel each other out. And then there is the players.
So, 5 players a team, each playing a unique character with its own special abilities and battlefield role. You get XP and cash from killing creeps, towers and other players. XP means more levels and abilities, cash means more gear, and the key to the game is to knock down the enemy towers, and then ultimately their base, which becomes easier if can "outlevel" the enemy characters, which is best done by killing them. Of course, tactically you can gang up and hit one lane and try and roll it up, or spread out to put pressure on across the map. You can concentrate on supporting your creeps to kill towers, or go hunting other players, or indeed some of the "unaligned" monsters dotted around the map.
DOTA2 is actually part of a florishing gaming sub-genre that is popular in the e-sports scene, and is essentially a remake of the original "mod" that first created it. This being value, they're attempting to make it as accessible as their big Free-to-Play success, Team Fortress 2, and also monetise it by community made skins for characters. It is a very slick game - matchmaking is quick and easy, the game is intuitive to play, all of which is pretty necessary given the rough time your first few matches can be.
A lot of the actual tactics of DOTA2 aren't immediately apparent. For starters, there is the vital nature of simply not dying to other players, which slows you down in leveling and of course boosts your opponents. Its a nasty recursive loop and the stuff of nightmares when you're on the wrong end of it. There is also sub-curves for each character and they combine with other characters, and a wider awareness of what is going on across the whole map. And of course, you are playing with, and often against, other people, which can be a whole new circle of Hell in itself.
But games usually last about 30-40 mintues, and after a good one, you want to stay on that roll. And after a bad one, you kinda want to try and wash the bad taste away. It's a game where the pace ebbs and flows through a match organically and unpredictably, where reversals and desperate, snatched victories are common. So, deceptively complex, challenging, fun, and best of all, it's Free!