Way back in the day, Lucifer was the first angel to fall from grace, and since then quite a few have followed her example.

Fallen angels are typically angels who have been banished from Heaven as punishment for a serious crime they've committed.

When banished, the angel will get a black halo with a red glow as well as black wings. Unlike other halos that serve as passports to get into Heaven, a black halo will keep you out of Heaven. The halo makes it hard to travel safely outside of Heaven because it's basically a big, glowing target telling demons and other hell spawn that 1) you're an angel and 2) you're alone.

Other angels are not allowed to interact with a fallen angel.

When banished, the angel is left to their own devices for any given time until they've proven they're worthy of coming back. Whatever that involves is very loosely defined, if defined at all.

If an angel gives up their halo and leaves Heaven for good they're no longer considered a fallen angel, but a devil.