You can combine units (into an army) if you have three units of the same type on one square. Units are much stronger in an army.
Pay attention to their offensive and defensive stats. Some units have great defense but weak offense and others are the opposite. Therefore some units are built to attack cities and units, while others are meant to defend cities and units. I should emphasize that you should protect your units. You don't want 5 armies of catapults to get crushed by a few warriors because they couldn't defend themselves, and had no defensive escort.
Also, you could actually win a game without ever fighting anyone, as long as you have the means to keep other civs off your back. Domination is only one way to win. You should be able to find much info on the web. You might not be able to find much for the WP version of Civ Rev, but other versions, such as the 360 version should have an abundance of info available after a quick Bing (Google) search.