Collections:
Collection is simply an object, that groups multiple elements into a single unit. Collections are used to store, retrieve, manipulate, and communicate aggregate data. Typically, they represent data items that form a natural group, such as a poker hand (a collection of cards), a mail folder (a collection of letters), or a telephone directory (a mapping of names to phone numbers). Finally I can say that Collection is Container.
If you Want to use the Apex programming language you need to know the collections. Collections are three types,