It is a big series of games, you have 3 main games Generals,Tiberian,Red Alerd Each of those have related games, the last released game was C&C4: Tiberiam Twilight, the first of the series that eliminated Resource Gathering, meaning you just build your army with some kind of auto-refreshing energy, it is a great game, but you lost one of the main itens in the series, all the others are focused on resource gathering, army, and some "heroes" missions.
Just Watch some movies about them and choose what is the one you will like best.
Generals is more like modern time.
Red Alert, i belive happens betwen times, time travelling if i'm not mistaken.
Tiberiam series are more close to starcraft, on some high developed planets.
Most of them have real "famous" actors in cutscenes, what gives a charm to the game...
there is already another game coming is the Generals series.
that is basically it.