Basic
Variance
$Var(X)=E[X^2]-E[X]^2$
Advance
PMF v.s PDF v.s CDF
PMF (Probability Mass function)
A PMF is a mathematical function that gives the probability that a discrete random variable is exactly equal to a specific value. It is used in statistics to describe the distribution of discrete random variables.
$p(x) = P(X=x)$, $p(x)$ is PMF