Collections Framework

A Collection is a group of objects about which no assumptions are made with regard to ordering or duplicate elements
A List is a Collection that has an ordering of its elements and allows duplicate elements
A Set is the opposite of a List, in that there is no ordering of the elements and duplicates are not allowed
These three interfaces are completely separate from their standard implementations
The major List implementations are ArrayList and LinkedList
The major Set implementations are HashSet, LinkedHashSet, and TreeSet
Closely related to the Collection interfaces is the Map interface
Java Collections page

It is common to create a reference to the interface type rather than the implementation type, although this is not necessary in general:

// interface types
List<String> names = new ArrayList<String>();  
Set<String> names = new TreeSet<String>();  

// implementation types
ArrayList<String> names = new ArrayList<String>(); 
TreeSet<String> names = new TreeSet<String>();

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