A random variable is a real-valued function defined on a sample space. Random variables are the main tools used for modeling unknown quantities. In statistical analyses. For each random variable X and each set A of real numbers, we could calculate the probability that X takes its value in A. the collection of all these probabilities is the distribution of X. There are two major classes of distributions and random variables: discrete (this section) and continuous (section 3.2). Discrete distributions are those that assign positive probability to at most countably many different values. A Discrete distribution can be characterized by its probability function (p.f); Which specifies the probability that the random variable takes each of the Different possible values. A random variable with a discrete distridbution will Be called a discrete random variable.