Karma, like gravity, is one of the basic rules of the universe... it's not a matter of belief. It applies to us all whether or not we "believe" it.
In it's original sense, Karma simply means "balance". Many Wiccans muddle it a bit and equate it with the Rule of Three (What you send out returns to you threefold), but this isn't the case. Karma isn't about being punished in this life for wrong deeds in a past life, or the opposite rewards for good behavior.
Instead, Karma is the force that seeks to find a balance in all things. Temperature is a good example. Hot and cold are two extremes of temperature, and they press air differently. Cold seeks heat, looking for equilibrium. On a hot day, opening the door to an air-conditioned store, the cold air rushes out the doors with a refreshing breeze. On a cold day, the cold air seeps into a house through whatever cracks it can find. The temperature is seeking balance. It doesn't mean that the heat is good or bad... neither is the cold. Temperature simply is.
Karma is like that. Karma is the force that seeks balance in all things. When it comes to the energy of the universe, which we take part in as we live our daily lives, Karma is the great equalizer. If our lives are way out of balance, Karma seeks equilibrium. If you are truly wicked, then your use of the life force is out of balance, and the force of Karma will apply pressure to modify your wickedness. If you are truly sacred, then you are living in balance with the universe, and Karma will support that harmony.
So although it's not quite as simple as "Do bad things, and Karma will get you", it often works out that way. Karma is not a "being" that wishes to punish or reward... it's simply a principle that exists in the natural world.
~Flame RavenHawk