Vectors
Vectors are one of the most-used Rust data structures. In other programming languages, they'd simply be called Arrays, but since Rust operates on a bit of a lower level, an array in Rust is stored on the stack (meaning it can't grow or shrink, and the size needs to be known at compile time), and a Vector is stored in the heap (where these restrictions do not apply).
Vectors are a bit of a later chapter in the book, but we think that they're useful enough to talk about them a bit earlier. We shall be talking about the other useful data structure, hash maps, later.