Humans, in Stephenie Meyer's world of vampires and shape-shifters are those that are born and do not have any of the unique features of mythical characters. Humans are often oblivious and don't have any idea that any of the olden-day traditional stories of vampires and "werewolves" (although strictly speaking, these are not what the La Push "wolves" are,) are real.
The main diet for vampires consists of the blood of humans, although some vampires and their covens have decided to officially take up the vegetarian lifestyle, drinking only the blood of animals. To most vampires, however, this just seems odd, as humans are preferred to animals because of the taste. Their natural enemies are shape-shifters.
Shape-shifters, such as those from the Quileute tribal legends, are normal humans, that turn into animals (wolves, in the Twilight Saga) when vampires pose a threat to the safety of their tribe or the people in the area. In the case of the tribe in Forks in relation to the Cullen vampires, a treaty was signed. Put simply, because of the Cullens "special" diet, as long as the Cullens do not bite a human or trespass/hunt on the Quileute's reservation, they shall not be hunted down.
Humans do not have any special abilities that the other creatures do in the series, although they age, change and bodies work as they naturally should do, which neither of the other species can.