Answer:
The energy released is a portion of the binding energy of the nucleus.
The binding energy of a fissile nucleus is higher than the sum of the binding energies of the nuclei that it splits into. This difference is what is released in the fission process. Some of it appears as kinetic energy of the particles, and some of it is emitted as electromagnetic radiation (typically gamma waves).
How a nuclear generating station works requires a long enough answer to be worthy of a separate question (also, there are several different types, each of which is slightly different).