Difference between time.time() and time.clock()

def fac(a):
    if a <= 1: return 1
    return a * fac(a-1)

def a(n):
    import time
    t = time.time()
    c = time.clock()
    for i in range(200, 900):
        f = fac(i)
        s = "Thread %d: %d -> (X)" % (n, i,) # fac(i))
        s += "@ t:%f c:%f" % (time.time() - t, time.clock() - c)
        print s

if 1:
    import os
    #Fork bomb alarm: 3 leads to 8 processes!
    #for i in range(3): os.fork()
    pids = [os.fork() for i in range(3)]
    print pids
    import time
    time.sleep(.7)
    # now the show begins:
    a(os.getpid())
else:
    import threading
    threads = [threading.Thread(target=a, args=(i,)) for i in range(10)]
    for t in threads: t.start()
    for t in threads: t.join()