Generally speaking, "etwas" can be (and usually is) shortened to just "was" in colloquial speech. You don't usually see this in formal speech or in writing.

for germans the word "etwas" means a undifined small quantity. "Iss was" -> "Eat something" , but "Iss etwas" sounds like "Eat a little bit". In generaly siebolt is right. The safe side is to use "etwas". That's nearly alway correct. :)

And if you want to stay on the safe side: always write etwas. But: in a question : Was ist das?