Learning to use the most appropriate class or object for the task at hand is fundamental to writing efficient code. This is especially true when dealing with collections.

Happily, there’s a document named Collections Programming Topics on Apple’s Developer Library that explains in detail the differences between the available classes, as well as which situations suit each class. It’s a must read document for anyone looking to work with collections.
TLDR? Here’s a quick synopsis of the most common collection types:

• Arrays: Ordered list of values. Quick lookup by index, slow to lookup by value, slow to insert/delete.
• Dictionaries: Store key/value pairs. Quick lookups by key.
• Sets: Unordered list of values. Quick lookup by value, quick to insert/delete.