• Battle - A Standard skirmish mode with alliances as a possibility.
• Free For All - Like Battle mode, except alliances are not allowed.
• Unholy Alliance - Each player starts the match with both an Allied and Soviet MCV (Mobile Construction Vehicle, used to build construction yards).
• Megawealth - Players must capture all the oil derricks with engineers in order to get money, because there is no mining allowed.
• Land Rush - Players begin the match in the middle of the battlefield, each with an MCV. There are power-up crates spread around throughout the map, and players must rush to get the crates.
• Meat Grinder - In this game, tanks and infantry are the main forces used.
• Naval War - Like you would think, the war takes place on several small islands, and naval power is very important.
• Cooperative - This is probably the most interesting mode of all, in my opinion. Each cooperative game (there are five), offers a small campaign of five maps, in which the humans fight against preset computer opponents.
• Global Domination: Players logged into Westwood Online will be able to participate in an ongoing struggle for control of a continent. This online feature was first introduced with the add-on mission pack Tiberian Sun:Firestorm.

Available Countries (Factions):

Allied: United States, United Kingdom, France, Germany, and Korea
Soviet: Soviet Union, Iraq, Libya, and Cuba

NOTE: Each faction gets it's very own special unit. Here's a list of the special units:

United States - Paratroopers
United Kingdom - Sniper
France - Grande Cannon
Germany - Tank Destoryer
Korea - Black Eagle

Soviet Union - Tesla Tank
Iraq - Desolator 
Libya - Demolition Tank
Cuba - Terrorist

For a more detailed description of the special units go here to the special units section.