An "assault" weapon is a vague term usually used to obfuscate the difference between military looking semi-automatic long arms and various automatic firearms. Wikipedia seem to have a decent discussion, although I didn't read all of it.
Here is an example of the stupidity
Weapon:
"Assault" Weapon:
Same model gun. The first picture is "Stock" minus the bayonet to make it legal for import. The second is filled with after market accessories and addons. Still missing the bayonet but the bi-pod uses the bayonet lug in this case so probably can't have both.
These are not my pictures, just ones I found on the net. I own the same model myself, but not pics sorry. And yes mine is technically "banned" because of my after market stuff, but I don't give a rats ass.