Prospective P/E divided by the estimated future growth in EPS
The important thing about the PEG is that both the earnings and the EPS growth are based on brokers' estimates for the next 12 months. i.e. they are comparing like with like. The PEG rule of thumb is that a company with a PEG of less than one is a bargain. But this is a complicated technique, and if it interests you, take the tutorial 'Growth Investing'.