This one made me laugh, so I thought I would try to answer it!
As far I as I can ascertain, the difference between a Goblin, an Elf and a Dwarf are as follows:
Of course, a dwarf can be many things. It can also be a type of star, which is both large or small. If something is dwarfed by something else, it has been massively over-shadowed by the other thing, so actually, dwarf doesn't always mean small (kind of an irony really to small people - bit like saying wicked when something is good and not evil?!
Some quality Dwarf related links are - Hank the Angry drunken Dwarf
How can any one forget the mental dwarf from Jackass Wee Man Jason Acuna. Visit his website to see a small man getting big boobed girls!
As for the other two, they are mystical characters where elves are nice and goblins are not - thats about it - chack the Wiki for definitions if you want to know more about elves, trolls and Norse Mythology