difference between binomeal distribution and poisson distribution?

Answer 1

You can approximate the binomial distribution with the Poisson distribution if the number of trials n is large and the probability p of any individual trial succeeding is small. The Poisson distribution is definitely the one to use if n and p are unknown separately but the average number of success np is known. Your Poisson parameter λ (lambda) or whatever symbol your text uses for this parameter, is the same as np.

Answer 2

A Poisson distribution is a special case of the binomial distribution when p<<1. I think there are other restrictions.