NPV is NET Present Value, it is a standard method to 
measure the time value of money to appraise long-term 
projects. Its also called as Net Present worth.

PBP is PayBack Period, measures the time required for the 
cash inflows to equal the original outlay. It measures 
risk, not return.

NPV tells the sum of all the present value if the cash 
inflows while payback period tells that How many years will 
it take for the cash inflows to pay the original cost of 
the investment.

cash in flows - cash out flows = NPV ( + or - )
PBP is the period at which the invested amount on the
longterm assets will get back by the company