Markov Chains
Markov Chains are a type of stochastic process (a collection of random variables that evolves over time) that satisfy the Markov property (the future state $n+1$ only depends on the current state $n$, and not any of the past states).
Markov chains are often used to model transitions between discrete states.
http://prob140.org/textbook/content/Chapter_10/00_Markov_Chains.html