Discrete random variable: A discrete random variable is one which can only take on distinct values. For example, counting how many red cars pass by your house would be represented by a discrete random variable.
Probability mass function: The probability mass function (PMF) gives the probabilities associated with each value that a discrete random variable can take. It specifies how likely each event is to occur.
Random Variable: A random variable is a numerical outcome of a random phenomenon. It assigns a unique value to each outcome in the sample space.