I'm going to guess that dwarves are short and stocky, while elves and goblins are long and lanky, and humans are like Shaq to all.
As for their origins, seeing as DF will eventually generate fantasy worlds rather than recreating a very specific one but with a different map, I'm going to guess the origin of species is ambiguous and left open to interpretation. The three basic conventions are creationism, standard evolution, and supernatural evolution.
Whatever party responsible for creating the world could have created them all as separate lineages. Dwarves, elves, and goblins could have split off from humans due to living with certain influences (demons, the underground, nature spirits, deities of various spheres, primal magical forces, etc. The other races could have come from various spirits or other supernatural entities interbreeding with humans, or are the descendants of such entities who took humanoid form or became anthropomorphic for whatever reason.
Elves could literally be descended from whatever natural force they worship, sprung from the soil as it's envoy to mankind or something. Dwarves could be likewise for the more mineral parts of the earth. Both could then have gone there own way after forgetting about the whole ambassador thing.