The AR-15 is an assault rifle (it was based on the AR-10) The M-16 series of rifles were adapted from the AR-15.I thought Biden called the AR15 an assault rifle that he wanted confiscated. Maybe you have correct information on that.
What you may mean is to distinguish between semi-automatic and automatic rifles. The newer AR-15's for public purchase do not have the burst option. So AR-15's can be assault rifles, but not all are assault rifles.
Per the Defense Intelligence Agency, “assault rifles” are “short, compact, selective-fire weapons that fire a cartridge intermediate in power between a submachine gun and rifle cartridges.” So the difference is (or could be argued) that an AR-15 without a burst function is not an assault rifle while an AR-15 with this function is an assault rifle.