The game that started the RTS genre
Lets start with the single player. You have two sides that are GDI, the Global Defense Initiative, and the Brotherhood of Nod. GDI are the good guys trying to maintain peace in the world while the Brotherhood is a fanatic group driven by their extremist loyalty to their leader, Kane. Kane's ruthlessness and charisma made him one of the most iconic villains and video game characters of the 90s. Both the GDI and Nod campaigns branch off in separate storylines and have different endings.
In these campaigns, we have the units you can train on both sides. GDI units focus on firepower as you have heavy hitters like the medium tank, mammoth tank, orca chopper while Nod units focus on speed like with the light tank, recon bike, stealth tank. Although GDI is heavily favored in a fight since their units pack more punch and Nod units are usually light armored, a Nod player can defeat a GDI player if they have strong micro abilities to maneuver their fast attack units.
There are a few drawbacks to the first C&C game. One is the lack of skirmish mode, unable to play a random melee game vs the computer AI. Another is the lack of usefulness of infantry. Infantry are slow and die easily. The only infantry worth building are the rocket soldiers. In masses they can easily destroy any tank or air attack, if they can stay alive. So for offense, you're only choice is to build vehicles, which has another drawback due to the building mechanics of the game. If you have one factory, you can build one unit at a time. If you have two factories, you can still only build one unit at a time but its build time is reduced. Even though the build time is reduced, the ability to only build one unit at a time forces the player to amass only one type of unit for the duration of the game. In the end, it'll most likely be just a tank vs tank battle, lacking variety and defeats the purpose of having counter units available at your disposal.
Other than those drawbacks, C&C was a defining game of the 90s and one of the best RTS games.