Barring sterile breeds, there are three out comes when two races breed, no child, a mixed breed, and one of the parent breeds.

Aside from elves, eladrin, orcs, and tieflings, none of the pair are delt with in the main books. I would probably say halfling's and humans just make short humans(and halfling and elves make short half-elves), and most other combinations do no produce viable offspring.