Sideburns can be worn alone, or in combination with moustache or goatee for example. When hair extends from ear to ear via the chin, the sideburns are merely part of a beard, and thus not called as such.
The name, originally burnsides, is probably derived from General Ambrose Burnside.
The native indians of Mexico would shave their heads and wear their sideburns long (called balcarrotas). This term is also used by Colombians who would wear their sideburns long and typically would not have any other facial hair (goatee or moustache).