Rules are rather simple.
The aim is to kill the other teams "Throne" or "Tree" depending if you're Scourge or Sentinel. There are three main paths with "wilderness" separating them. Along each path are towers which are like out posts, or strong points. Good places to defend etc, but good rewards to destroy too.
Along each of these paths creeps spawn, like every 15-20 seconds a new wave will spawn and walk down the path towards each other. They will attack the first enemy they see and attempt to "push" the path the opposite direction and eventually destroy the enemy towers and finally their base.
At the start of the game you can choose one of about... 80-90 heros. And they're all pretty well balanced (this game has been around for years and is more popular than norm WC3 now). Each have their strengths and weaknesses. Everything from big spell nukes, melee DPS, mass disables, summoning, healing etc, to a combination of them all.
You get experience and gold for killing creeps, you get A LOT of experience and gold for killing other heros and towers. You can then build all sorts of items to help you. Combine type X, Y and Z and you'll get uber power item D.
The end result is a very diverse and ever changing game. No two games are the same. The combination of heroes are immense, and so too are the combination of items that can be employed for each of them.
It's a game that noobs can jump on, kill some people and enjoy, but at the same time if you've been playing for years (like me) you can still find a challenge, learn and improve.
Games last about an hour at most.
Just try it, I promise you you won't regret it.