From BR Bullpen
A starting pitcher (aka starter) is a pitcher who specializes in starting games. Starters are not expected to pitch every day. Instead, teams have a starting rotation of 5 starters, each of whom starts a game in turn. The starting rotation is expected to pitch the majority of a team's innings, with only a minority going to the relief pitchers in the bullpen.
Like all pitchers, starters benefit from having good stuff and control. Additionally, a starting pitcher is normally expected to pitch deep into a game, so he must have the endurance to throw 100 or more pitches at or near maximum effort. Because the starter is expected to face the same batters several times each game, he also must have a wide repertoire of pitches. A reliever may be able to get away with just one or two good pitches, but a starter is expected to have three, four, or even more usable pitches in his arsenal so that he can pitch differently to a batter each time he faces him. The major exception to this rule is that knuckleball pitchers can get away with throwing the knuckleball almost exclusively.