Probability distributions can be like screwdrivers in mathematical form. They handle specific problems really well. There is one which can help you estimate the number of people that will follow a link in your next e-mail, another that can help you estimate the demand on your servers, and yet another to help with finding the expected return of a particular type of investment.
Because there are many different types of problems, we have many different types of probability distributions. It can be hard to keep track of them. Here is a quick reference to help with of some of the more prevalent distributions. This graphic shows both the formula and what these distributions are usually used to model. It also shows the relations between these distributions to help you find just the right one.