Speed: Bullet to Correspondence Rating:
1. c4 is a flank thrust that establishes control of the d5 square. It also allows the Bishop on c1 and the Queen on d1 into the game.
By delaying the development of the other pieces, White retains flexibility and often this will transpose to other openings. For example, if d4 is played later in the game it becomes a Queen's Gambit.
Though not as popular as the central pawn pushes, 1. c4 has been employed by quite a few notable chess players, including Mikhail Botvinnik, Bobby Fischer, Anatoly Karpov, Garry Kasparov, and Magnus Carlsen.