what's the difference between f(a) and f(*a)

[qxyuestc at yahoo.cn]

def sum1(*a): return(sum(i*i for i in a))
def sum2(a): return(sum(i*i for i in a))

a=[1,2,3]
print(sum1(*a), sum2(a))
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showed above: the result from sum1() and sum2() is the same. So, what
is the difference between f(a) and f(*a)