# 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][collections].

[collections]: ../../std/collections/index.html

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