Swift provides three primary collection types, known as arrays, sets, and dictionaries, for storing collections of values. Arrays are ordered collections of values. Sets are unordered collections of unique values. Dictionaries are unordered collections of key-value associations.
Arrays
An array stores values of the same type in an ordered list. The same value can appear in an array multiple times at different positions.
Sets
A set stores distinct values of the same type in a collection with no defined ordering. You can use a set instead of an array when the order of items isn’t important, or when you need to ensure that an item only appears once.
Dictionaries
A dictionary stores associations between keys of the same type and values of the same type in a collection with no defined ordering. Each value is associated with a unique key, which acts as an identifier for that value within the dictionary. Unlike items in an array, items in a dictionary don’t have a specified order. You use a dictionary when you need to look up values based on their identifier, in much the same way that a real-world dictionary is used to look up the definition for a particular word.