It is a spectrum.

There is one Force, the difference is how you approach it, what you want to get out of it.

The light side focuses on controlling selfish desires for the benefit of everyone. The dark side focuses on gathering as much power as possible for the benefit of the individual with that power.

Because The Force represents so much power, it is almost impossible to use it in a non-selfish manner unless you have a strong motivating reason not to. A good analogy is a $1,000,000 daily allowance. What would you do with it? Would you use it to get what you want? to help people? to steal things you can't buy? The Jedi believe that people, especially those who cannot control their emotions and desires, will eventually start using their power for selfishness. This is why they always talk about the light and dark side as absolutes, it is almost impossible to be neutral.

Anakin is a good example. While he wanted to do what was right, his primary desires were to get what he wanted. This was shown when he gets angry about how long it took to get on the Jedi council, when he attacked the sand-people villagers when his Mom died (rather than finding out why they were acting strange) in order to exact his revenge, and the whole ending of Episode III. He knew what he wanted and he did not care who he hurt or the amount of damage he would do, as long as he was able to get what he wanted. It was his strong, uncontrolled emotions which pushed him to fight so hard for what he wanted (despite the destruction), which is why strong emotions like love, hate, anger, envy, and fear are counseled and guarded against by the Jedi, but embraced by the Sith.

It is a spectrum.

There is one Force, the difference is how you approach it, what you want to get out of it.

The light side focuses on controlling selfish desires for the benefit of everyone.
The dark side focuses on gathering as much power as possible for the benefit of the individual with that power.

Because The Force represents so much power, it is almost impossible to use it in a non-selfish manner unless you have a strong motivating reason not to. A good analogy is a $1,000,000 daily allowance. What would you do with it? Would you use it to get what you want? to help people? to steal things you can't buy? The Jedi believe that people, especially those who cannot control their emotions and desires, will eventually start using their power for selfishness. This is why they always talk about the light and dark side as absolutes, it is almost impossible to be neutral.

Anakin is a good example. While he wanted to do what was right, his primary desires were to get what he wanted. This was shown when he gets angry about how long it took to get on the Jedi council, when he attacked the sand-people villagers when his Mom died (rather than finding out why they were acting strange) in order to exact his revenge, and the whole ending of Episode III. He knew what he wanted and he did not care who he hurt or the amount of damage he would do, as long as he was able to get what he wanted. It was his strong, uncontrolled emotions which pushed him to fight so hard for what he wanted (despite the destruction), which is why strong emotions like love, hate, anger, envy, and fear are counseled and guarded against by the Jedi, but embraced by the Sith.

One additional "side" is mischief. While not as strong a drive for most humans, a Loki or Puck character who acts to bring change and "keep things interesting" would not fall into the self serving nature of the dark side or the selflessness of the light side. Those in this category would technically be gray since they can do light and dark side things, but they are not defined by those actions (their light or dark side choices cannot be used to predict future decisions).