The left fielder covers the left portion of the outfield grass (when viewing the field from home plate).
On defense: Speed and range for a left fielder aren't considered as crucial as they are for a center fielder. Additionally, left fielders typically don't require as strong of a throwing arm as center fielders or right fielders, as throws from left field to second, third and home -- particularly third -- are not as long as throws from center field and right field.
On offense: Because left field is among the least demanding defensive positions in baseball, left fielders are typically expected to be offensively viable. If a player is a strong offensive asset but has questionable defensive skills, a club may play him in left field to keep his bat in the lineup.