The National Planning Policy Framework (NPPF) is the governments overarching planning policy document. This document sets out the planning policies for England and how they are to be applied. It provides guidance for local planning authorities in terms of determining planning applications and producing local plans. The NPPF should be read in conjunction with the Planning Practice Guidance document, you can view or download both documents below:
The government has produced a Plain English Guide (pdf) to the planning system in England which may also be of use.