One of the best aspects of Red Alert 2 is the ability to compete against opponents from all over the world, of all abilities. You can compete in ranked or unranked games and form clans to compete against other clans. The service used is XWIS - there is a ranking system that you can play in. Thousands of people play in the tournament system. XWIS became the official online service for most C&C games in October 2005, replacing the now extinct Westwood Online (WOL).
In multiplayer mode you are allowed to pick any of the countries (just like on skirmish) - here are the eight countries and their respective units:
France: Grand Cannon
Germany: Tank Destroyers
Korea: Black Eagle's
Russia: Tesla Tank
Libya: Demolition Truck
Cuba: Cuban Terrorist
You can see the special units description page here.
There are several different modes of play on the internet:
Battle: This is the default mode; the one you play in skirmish.
Unholy Alliance: You start with an allied MCV and a Soviet MCV.
Naval War: Emphasis on naval war... no high tech units save for naval ones may be made.
Meat Grinder: No high tech units allowed - intended for tank V tank warfare.
Mega Wealth: You must capture and defend oil derricks instead of harvesting ore.
Co-Op: You play one of a set of missions with a friend against the computer.