Revan was a force user, one of the strongest and on who according to cannon rejected both the Sith and the Jedi. There are MANY examples in the Expanded Universe books of Force users individual and groups that are neither Sith or Jedi.
The Dark side as been said is channeled through negitive emotions. Anger, Hate, and Fear. Sith encourage and use these emotions to channel the force. Of course using it in such a way will magnify those emotions and corrupt a person.
The Light side chooses to channel the force only through peace, very buddah. They do so not to reject all emotion (Luke Skywalker gets married, has kids, feels love) but recognizing that using emotion to channel the force magnifies that emotion, makes it stronger. And emotions can always cloud judgment.
The other force users/groups out their just use the force, in some unique ways. They don't worry about emotion or light/dark. Light and Dark is almost an exclusive Sith/Jedi idea. To them there is just the Force. You might be a good person you might be a bad but its all on you what you do. There is just the Force.
