A collection is a group or store of data that has a shared significance. Swift provides two native collection types for storing data. These collection types are known as arrays and dictionaries. An array will store the data in an ordered list, while a dictionary will store the data in an unordered list. In an array, we access the data by the location (index) in the array, while the data in a dictionary is accessed using a unique key that is associated with the value we want.

The data stored in a Swift collection is required to be of the same type. This means, as an example, that we are unable to store a string and an int in the same array. Since Swift does not allow us to mismatch data types in a collection, we can be certain of the data type when we retrieve data from a collection. This is another feature, which on the surface might seem like a shortcoming, but is actually a design feature that helps eliminate common programming mistakes.