A Sith is a follower of a Force tradition bordering on a full-fledged religion that seems to be centered around being the opposite of a Jedi: merciless instead of merciful, cruel instead of compassionate, and--from a very "cheesy" perspective--evil instead of good. After a certain point in galactic history, no more than two Sith (Darth Vader and Darth Sidious) ever exist again, and at the end of the Battle of Endor, the last two Sith Lords are killed and most (if not all) of the formal texts and learning of the Sith Order is lost.
"Dark Jedi", meanwhile, is a catchall term for any Jedi or individuals with Jedi training who fall to the Dark Side. Desann clearly falls into this category, and though he may have found something left over after the main Massassi temple on Yavin IV was destroyed by Corran Horn that instructed him in some esoteric Sith methods, his main source of power and information was the Valley of the Jedi, and he was not a Sith. As well, any number of Dark Jedi, according to various EU sources, may have served the Emperor, including Asajj Ventress. (An interesting aside: Asajj apparently considers herself Sith; Darth Tyranus is amused by and disdainful of this assumption.)
"Dark-sider" is a term used for any Force-user who turns to (or is raised in) the Dark Side, wasn't a Jedi, and isn't a Sith. The Prophets of the Dark Side, Tavion (apparently), the Nightsisters of Dathomir, and so on and are dark-siders.
Ah, yes, the lightsaber. The weapon of the Jedi Knights, those guardians of peace and justice. Has anyone noticed that guardians of peace and justice in sci-fi and fantasy tend to wield BLADES of DEATH?