Human (Generic)

A generic human is a member of a sexually dimorphic, bipedal apelike sapient species that walks upright and plantigrade, is mostly hairless, has a comparatively big brainbox for its size and is known for complex social interactions (usually).

Aw heck, just read [Wikipedia's article on humans].

Purpose in fiction

Fantasy

In fantasy, humans are often depicted as the 'normal' species used for contrast with superior elves, wacky fae or those damn' dirty goblins, orcs and trolls.

The reader is usually invited to identify with a human character while discovering the setting through his or her eyes.

Science fiction

In science fiction - particularly, it seems, that originating in the USA - humans are often depicted as the most balanced and wise species among many alien races whose personality and culture is defined by one single human attribute or emotion amplified ad absurdam. (You know; like [greed], [intolerance], [aggression] or [detachedness].)

Alternatively, sci fi can follow the "humans are as children compared to the superior elves" format, where elves are substituted for a similar wise 'big brother' alien species.

As in fantasy, humans or human characters are often used as a peg with which the audience can identify.

Speculative fiction in general

Humans are also often cast as the dupes or innocents from whom magic, the supernatural/paranormal, government genetics experiments or other worlds are kept secret. You might infer a certain amount of paranoia on the part of fantasy and science fiction writers as a whole.