Covert Operations is a official expansion set for Command & Conquer. Covert Operations II is a non-official modification set for Command & Conquer: Tiberian Sun.
_______________________________________________________________________________________

Mod enhances the gameplay by adding more units/buildings/weapons and changing other variables, which will make gameplay more fun.
Some Mods feature very little changes only, such as a few new units and buildings, and some mods features a lot, like adding new missions and lot of units.
____________________________________________________________________________

In order to play mods, you must copy/unzip all require files to your Tiberian Sun Directory. 
Some Mods may contain a Launcher, which can activate and de-activate the Mod Automaticly.
___________________________________________________________________________ 

Sure you can play with your friends with multiplayer option, but they must also have the same mod.
Note: You MUST remove all mods before playing in Westwood Online or you will be kicked out of game.
_______________________________________________________________________________________