For those who are familiar with Objective-C, you will know that there are different classes for mutable and immutable collections. For example, to define a mutable array, we use the NSMutableArray class, and to define an immutable array, we use the NSArray class. Swift is a little different, because it does not contain separate classes for mutable and immutable collections. Instead, we define whether a collection is constant (immutable) or variable (mutable) by using the let and var keywords. This should seem familiar by now since we define constants with the let keyword and variables with the var keyword.