The Force was a form of mystical energy, that bound the fabric of reality together in the Star Wars Multiverse. It helped life sustain itself in a sort of symbiotic relationship. When one was harmed, then so was the other.
It is important, that in the vast majority of universes where The Force is present, that it has a light side, and a dark side. Two religions formed around each side, the Jedi and the Sith respectively. While the Jedi were mainly concerned about keeping the peace and bringing balance to the universe (In most cases, anyways), the Sith were predominately concerned with using The Force for their own gain, using it to garner, fear, respect, and power. Some say that Sith ambition led to the unbalancing of The Force and harm to the universe, this appears to be not much more than Jedi propaganda.
Regardless, The Force is a powerful weapon, granting the bearer telekinesis, levitation, limited mind control, space/time distortion, and even the ability to bring about life.