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 calculeat 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: There are two major classes of distributions and random variables: 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.