The Poisson distribution is a discrete probability function that is used to calculate the probability of a number of events occurring in a specified time period.
The Poisson probability mass function calculates the probability that there will be exactly x occurrences and is given by the formula:
where λ is the expected number of occurrences within the specified time period.
The cumulative Poisson distribution function calculates the probability that there will be at most x occurrences and is given by the formula:
Further information on the Poisson Distribution is provided on the Wikipedia Poisson Distribution PageThe Excel POISSON.DIST function calculates the Poisson Probability Mass Function or the Cumulative Poisson Probability Function for a supplied set of parameters.
The function is new to Excel 2010 and so is not available in earlier versions of Excel. However the Poisson.Dist function is simply an updated version of the Poisson function, which is available in earlier versions of Excel.
The syntax of the Poisson.Dist function is:
Where the function arguments are:
x    The number of events that you want the probability for (must be ≥ 0). This value should be an integer; If a decimal is supplied, it will be truncated to an integer by Excel.  
mean    The expected number of events (must be ≥ 0).  
cumulative    A logical argument that specifies the type of distribution to be calculated. This can be either:

The above chart on the right shows the Poisson Probability Mass Function with an expected value (mean) equal to 25.
If you want to calculate the value of this function at x = 20, this can be done using the Excel Poisson.Dist function, as follows:
This gives the result 0.051917469.
The above chart on the right shows the Cumulative Poisson Probability Function with an expected value (mean) equal to 40.
If you want to calculate the value of this function at x = 35, this can be done using the Excel Poisson.Dist function, as follows:
This gives the result 0.242414198.
For further information and examples of the Excel Poisson.Dist function, see the Microsoft Office website.
If you get an error from the Excel Poisson.Dist function this is likely to be one of the following:
#NUM!    Occurs if either the supplied value of x, or the supplied mean is < 0. 
#VALUE!    Occurs if any of the supplied arguments are nonnumeric. 