Command & Conquer is a Real-Time Strategy (RTS) series originally developed by the former Westwood Studios before being acquired by EA and subsequently passed off to an EA studio, with mixed results. 

The first game, originally titled "Command & Conquer" and later subtitled with "Tiberian Dawn", puts the player in the shoes of a military commander in the late 90's, after a meteor lands in the Tiber River in Italy, unleashing the dangerous, virulent material named "Tiberium." The Brotherhood of Nod, an ancient secret society led by an enigmatic man known as Kane, is able to make the technology to exploit Tiberium, giving them the upper hand. This leads to them growing into an international terrorist force, prompting the UN Security Council to send in the Global Defense Initiative to stop them. 

Released in 1995, the game became one of the most iconic RTS games in history, cementing a dedicated fanbase for years to come. 

The second game in the Tiberium series, Tiberian Sun, released in 1999, took place in the year 2030, in a world where GDI won in the first game. Things take a turn for the extra-grimdark, as Tiberium has evolved and spread across the earth to an alarming degree, Nod is still around, and Kane seems to have come back from the dead. Technology has advanced to quite a degree, as the game features bombers that shoot plasma torpedoes, tanks that fire soundwaves and an AT-AT reference in the form of the Mammoth Mk. II. The game received an expansion, "Firestorm", to fix the initially unbalanced and feature-lacking game and add more story, the plot of which being that Nod's AI, CABAL, develops a will of his own, gets captured by GDI so that they can use him to decipher the series' MacGuffin, the Tacitus, said to contain a wealth of knowledge about Tiberium. Firestorm was notable in that both factions' storylines were canon, since they were interwoven into one big thread of plot.

The sequel, Tiberium Wars, released in 2007, takes place in 2047, in a world where GDI has become a global superpower and has managed to fight off the encroachment of Tiberium to a degree, despite its evolution, using data from the Tacitus. The Brotherhood of Nod decides to make a hell of a comeback, and Kane once again comes back from the dead. Like in Firestorm, both sides have their plots interwoven into one story.

The last game in the Tiberium series, Tiberian Twilight, released in 2010, was a radical change from the usual formula of the series, to put it mildly: in place of the usual economy and mobile construction vehicles were Crawlers and population caps and a multiplayer mode inspired by Unreal Tournament's Domination mode. In it, Tiberium has evolved again and it seems as if the earth is doomed...until Kane approaches GDI with an offer to build a "Tiberium Control Network" that will contain all the Tiberium on earth. All seems to go well until a separatist faction led by a guy called Gideon decides to ruin everything, so it's up to the Commander to once again pick a side (which also involves an interwoven plotline) and follow in the machinations of that side's leader. The game was poorly received mainly because of the radically different gameplay and disappointing ending, marking a sad conclusion to a legacy.

There is also a side series, known as Red Alert, and there's that one game called Generals, but this article is long enough already.