Vampires: they were previously humans but they were bitten with the vinom of vampires but without losing all their blood thus they didn't die but they became immortal.
Werewolves: they are descendends of Indians or something in the story thats why you find Jacob and his people all tanned.
Humans: you , me "the real people"
they are all related in terms of a contradictory relationship, for example Edward and Jacob have a treaty that neither of them comes to the other's land, and that they hate how the other smells, and they have been enemies since forever.
as for humans the werewolves are humans and they don't hurt humans, they can marry humans and they can transform their shape into humans so the humans and the werewolves have lots of similarities.
as for vampires well... you can say they are basically the outsiders, werewolves hate them, humans are afraid of them, and in the case of Twilight, the Cullens are a "vegitarian" family in their own sense; meaning that they don't drink human blood because they don't want to hurt humans but they drink animal's blood, thus in the story the Cullens are somewhat close to humans and that is why they were able to live with them and be around them but for the new ones it was hard.