Chances are, they look pretty much like you. Their skin varies from alabaster to a rich dark ebony brown. Despite what certain segments of the human population wish to believe, there are no significant natural differences between the various subgroups of humans other than skin color. That's not to say there aren't significant cultural differences, but that's a completely different story.
Humans are all around average. Adult human heights average 5'7" for females and 5'11" for males, but the normal range is anywhere from 4'8" to 6'6" with some rather extreme outlying heights (2'3" and 8'3"). There's a striking similarity between elves and humans except that humans are less adept at magic, have shorter average heights, slightly heavier average builds, have rounded ears, live shorter lives, and heal less quickly. That's not to say there are no benefits to being human. On the contrary, elves are often jealous of humans! Humans are better adapted for the increasingly mechanized world and have mechanical skill rivaled only by the goblins. The human mind is also capable of picking up and learning any skill, trade, or language. There truly is no limit to what they can learn.
There's a subset of humans called men that are stronger and smarter than their counterparts.