${n \choose k}$: the number of $k$-length subsets of a set with length $n$.
Multisets: Sets without uniqueness. $t$ types of (repeatable) elements.
Number of multisets of length $n$ is ${n + t - 1 \choose t - 1}$.
Surjective: Onto. Every element in the codomain is the image of at least one element in the domain.
Injective: One-to-one.
Bijective: Both. Wires task in among us.