1. The document defines discrete random variables as random variables that can take on a finite or countable number of values. It provides an example of a discrete random variable being the number of heads from 4 coin tosses. 2. It introduces the probability mass function (PMF) as a function that gives the probability of a discrete random variable taking on a particular value. The PMF must be greater than or equal to 0 and sum to 1. 3. The cumulative distribution function (CDF) of a discrete random variable is defined as the sum of the PMF values up to that point. It ranges from 0 to 1 and increases monotonically.