Crap is a versatile word, usually meaning feces, stuff, or garbage. It's definitely slang but not really obscene.
This is a word that's on the borderline between acceptable slang and potty language. You can be 100% sure that saying the s-word (which means almost exactly the same thing as crap) will get you in trouble if you said it in class, but crap isn't so clear cut. It's sort of naughty but getting less naughty all the time. But whether you're taking a crap (pooping) or being told to get all the crap (junk) off your desk, you'll find that crap of any kind is never held in high regard.