okie, you know your binomial distribution, and the relationship
between this and poisson right ? ...
P(X = x) = p(x) = lambda^x . e^-x/x!
suppose you want to find the probability of an event x, which can
take on only a fixed range of values, say x = 1/2/3 .... over a
large "landscape", e.g. the density of the number of bacteria on a
tiny area of a petri dish, or the density of the number of trees in
a square meter of jungle ... you use the poisson distribution ....
lambda would be the average value of your value space, the average
number of bacteria, or the average number of trees etc ...
mean and standard deviation is easy to remember .... it's just
lambda ... 
how I remember the application of this was to drill this into my
head: big landscape, small area, small value space, what's the
probability of it taking on a certain value ...
questions would ask you, suppose the average value of something is
this, what is the probability that it would take on a certain value
X ....
good luck bro ... 
