Card shuffling and $P$-partitions

In this expository article, we highlight the direct connection between card shuffling and the functions known as $P$-partitions that come from algebraic combinatorics. While many (but not all) of the results we discuss are known, we give a unified treatment. The key idea is this: the probability of obtaining a permutation $\pi$ from shelf shuffling is the probability that a random $P$-partition is sorted by $\pi$, and the probability of obtaining $\pi$ from riffle shuffling is the probability that a random $P$-partition is sorted by $\pi^{-1}$.