# Common Collections

Rust’s standard library includes a number of really useful data structures called collections. Most other data types represent one specific value, but collections can contain multiple values. Unlike the built-in array and tuple types, the data these collections point to is stored on the heap, which means the amount of data does not need to be known at compile time and can grow or shrink as the program runs. Each kind of collection has different capabilities and costs, and choosing an appropriate one for the situation you’re in is a skill you’ll develop over time. In this chapter, we’ll go over three collections which are used very often in Rust programs:

- A vector allows us to store a variable number of values next to each other.
- A string is a collection of characters. We’ve seen the `String` type before, but we’ll talk about it in depth now.
- A hash map allows us to associate a value with a particular key. It's a particular implementation of the more general data structure called a map.

To learn about the other kinds of collections provided by the standard library, see the documentation.

We’re going to discuss how to create and update vectors, strings, and hash maps, as well as what makes each special.