The Jedi and the Sith are most similar in their ultimate goal- to become one with the Force. What causes them to be different are their reasons and methodsfor getting to that point.

The Jedi seek to help others; the Sith seek to harm. Jedi seek to serve and protect; Sith seek to dominate and hurt. The Jedi get their power from the life within themselves connecting to the life without; the Sith, from emotions, especially negative ones. Jedi see themselves as guardians of the galaxy, while Sith see themselves as masters and overlords of it.

The Jedi train many Jedi, as many as is feasible, for the more Jedi, the stronger the order will be, and the more benefit they will bring to the galaxy. There are only ever two Sith, because they would destroy themselves, and they believe that they gain power by not sharing it. Jedi see the Force as their guide, and themselves as a conduit for the power of the Force, while the Sith see the Force only as a tool, something to be manipulated at will, bent into their servitude.

Both the Jedi and the Sith use the Force, but the Jedi use it for good, while the Sith use it impulsively for their own whimsy. The Jedi listen to the councel of the Force to determine what is the right, or at least the better, action, while the Sith train to use skills meant only for harm and destruction. The Sith rule through their own emotion and the emotions of others, while the Jedi acknowledge their own emotions and those of others, but do not allow them to influence their choices.

Ultimately, in spite of all these differences, the Jedi and the Sith are really quite similar. The main thing that distinguishes the one from the other is their intent and the means they employ to achieve a goal.