Centre-backs stop the main central attacking threat, win aerial duels and block shots. Modern centre-backs also contribute to the build-up: splitting wide to invite a press, carrying the ball into midfield or playing line-breaking passes.
In a back four they typically share marking duties; in a back three they split into a left CB, a right CB and a central CB who often functions as a sweeper.
A good centre-back reads the game better than he runs: positioning is worth more than raw pace, although extra speed helps when the team plays a high line.