In a database, what is the difference betwen a key and an index?

Question

For example, in SQLServer, if I have an index with \'unique\' set on it, how is that different from a key?

How do I know when I want to use a Key vs. an Indexed field?

Answer 1

Unique is a constraint forced on your data, whereas an index will help in ordering your rows physically (clustered index) or logically (non-clustered index) in the database for faster retrieval.