From BR Bullpen
The pitching rotation or starting rotation is comprised of the starting pitchers (usually 5) that a team plans to use over the course of the season. The pitchers in the rotation have higher stamina than relievers and, generally speaking, a wider variety of pitches. They are expected to pitch every 5th game. The rotation is set in a descending order of the pitchers' effectiveness. The top 1 or 2 spots of the rotation are generally given to the most talented pitchers - the "aces" of the staff. The 5th pitcher of the rotation may be skipped over occasionally, especially in the first half of the major league season, when there are more off days between games. Some teams even forego the 5th pitcher altogether, using a 4-man rotation in tandem with occasional starts from a long reliever.
The four-man rotation was the standard in the 1950s and 1960s, when teams began to play fewer doubleheaders and mor predictable schedule. The Los Angeles Dodgers were the first team to use a five-man starting rotation in the early 1970s - the pitchers were Tommy John, Don Sutton, Burt Hooton, Rick Rhoden and Doug Rau - and were soon imitated by all other teams. However, the five-man rotation was used before that, beginning in the 1920s, although the large number of off-days and doubleheaders made it less strict than the version used nowadays. There have been a few exceptions to this trend however, the most notorious being the 1972 and 1973 Chicago White Sox who used a three-man rotation (Wilbur Wood, Stan Bahnsen and Tom Bradley) with an occasional fourth starter.
Ironically, the White Sox began experimenting with a 6-man starting roation in May of 2011, as a result of circumstances. With the return of Jake Peavy to health, they found themselves with six healthy starters, and decided to give them all a turn. The move worked well and they were imitated six weeks later by the Kansas City Royals. The objective of the move is to limit the innings pitched by each starter in the absence of a workhorse starter; the downside is a very short bench.