If you've played Warcraft 3 online, you probably popped into a game of DoTA.
This game is a dedicated, free version of DoTA. Control 1 growing champion, work as a team of 3 or 5, in a battle against an equal enemy side.
The team that pushes to the enemy Nexus (command center) wins. All buildings are AI controlled, so you can't order anything from them.
It would kind of (in the barest sense) be like playing 5 on 5 Diablo with AI units supporting both sides. You have roles, tanks take damage so that the carries and casters can destroy the enemy without taking much damage. Support champions assist the others with their spells, so by working together you have a better chance of winning.
It's Shen free week so I have been playing for a while. It was Rammus free week last time. To break them down...
They are both "tanks" designed to guard allies and prevent the enemy attacking them.
Shen is a ninja. He can help allies heal, has a manually activated shield, a dash/taunt that goes through walls AND makes enemies attack him.
His ult (level 6 ability) is Stand United. Cast it on an ally who is getting beat up (anywhere on the map, even at the other end) and they will get a damage shield, then after a few seconds, Shen teleports to your target's location. Great for securing an escape for a losing battle.
Rammus is a sentient armadillo in body armor, who calls himself the Armordillo.
He has a roll ability call Powerball, which speeds him up and causes him to knock units in a group if he bashes into anyone while Powerballing.
He also has Defensive Ball Curl and Puncturing Taunt. DBC is a defensive stance, he gets more armor and resistance, as well as hurting people who hit him. Puncturing Taunt is a 1 target taunt that also lowers the armor of an enemy. So taunt near an allied turret, use DBC and watch as the enemy hits you, making the turret blast him, as well as the damage he takes from his own attacks.