I really have no idea what PyPy is. All I know is that it is an implementation of Python written in Python (?) and that it runs faster than Python. For example, I tested the Monty Hall Problem that I wrote recently, and timed them. Here is the result:
[daniel@TimeMachine Python]$ time python2 monty_hall_switch.py All choices were switched. Wins: 66679 Losses: 33321 real 0m1.265s user 0m1.243s sys 0m0.023s [daniel@TimeMachine Python]$ time pypy monty_hall_switch.py All choices were switched. Wins: 66853 Losses: 33147 real 0m0.594s user 0m0.477s sys 0m0.070s
So PyPy ran the program more than twice as fast as Python. Pretty neat, huh?

