PRA can express arithmetic propositions involving natural numbers and any primitive recursive function, including the operations of addition, multiplication, and exponentiation.

As a Skolem theory

Let $T$ be a Lawvere theory, i.e., a category with finite products $T$ equipped with a bijective-on-objects product-preserving functor $X: Fin^{op} \to T$ where $Fin$ is the category of finite cardinals and functions. Recall that a Skolem theory? is a Lawvere theory $(T, X: Fin^{op} \to T)$ in which $N = X(1)$ carries a structure of parametrized natural numbers object in $T$.

Abstractly, PRA can be described as the initial Skolem theory. Precise statements to this effect are difficult to pin down in the literature. (More later)