You can play WCS against a computer controlled opponent. Actually this is
what it was designed for, serving as a strong opponent based on its internal
artificial intelligence. However via (internet) network connections you can
play against other (human) opponents. This works in the following manner:
When playing WCS you get to choose a country from a world-map. This is the country where you will start to "liberate" the rest of te world. When you attack a neighbouring country you will not know (in case of a networked session) if your oponent is computer controlled or human controlled. However, computer controlled vehicles are plain grey, whereas human players can select other colours for their vehicles.
By clicking on (human controlled) enemy vehicles you'll have immediate contact with the player on the other side. You can communicate with you enemy via a chat-like window, or using a microphone connectod to your computer.