In baseball, corner outfielders are outfielders who play the corner positions of right field and left field. Corner outfielders often have less speed than center fielders, but make up for this deficiency with their bats. The main differences between left and right fielders are, first, that left fielders handle more chances because right-handed pull hitters tend to hit balls to left; second, that right fielders typically have stronger arms; third, that right fielders are frequently (not always) slower and less agile defensively. Many left fielders have had the speed to play center field, but have lacked the throwing ability required.
An example of an ultra-fast left fielder is Rickey Henderson (Ben Oglivie can fit this description too), whereas the slow-footed but very strong-armed Carl Furillo, "The Reading Rifle," sets a standard for right fielders in the terms specified here.
Gorman Thomas is an example of the reverse theory. He was a centerfielder his entire career (mainly with the Milwaukee Brewers), but was not nearly in shape as the typical player for this position. He compensated for it with sheer hustle and determination.
Often, when an outfield prospect arrives in the majors, he plays both corner positions before settling at one or another.