Collections

Lists
A list is an ordered collection of elements that are distinguished by their indices. List elements can be of any data type—primitive types, collections, sObjects, user-defined types, and built-in Apex types.

Sets
A set is an unordered collection of elements that do not contain any duplicates. Set elements can be of any data type—primitive types, collections, sObjects, user-defined types, and built-in Apex types.

Maps
A map is a collection of key-value pairs where each unique key maps to a single value. Keys and values can be any data type—primitive types, collections, sObjects, user-defined types, and built-in Apex types.

Parameterized Typing
Apex, in general, is a statically-typed programming language, which means users must specify the data type for a variable before that variable can be used.

Apex Developer Guide

See Also