What is the Will to Power?

To answer this question properly would be to discuss almost all of the themes of Nietzsche's philosophy. The WTP, as N. understands it, is the underlying explanatory cause of both natural and human processes. In the latter case, the WTP can exhibit diseased as well as healthy forms.
Much of his writing is an attempt to see how far human phenomena could be explained as manifestations of the WTP; consequently what he meant by the term can only be understood (he was not a systematic philosopher) by examining his examples provided throughout his writings.

the fundamental driving force behind all things

The selfishness that resides in everyone.

that might be incorrect

Will To Power is a naff 80s band.