What is an index in SQL?

Question

What is an index in SQL? Can you explain or reference to understand clearly?

Where should I use an index?

Answer 1

If you're using SQL Server, one of the best resources is its own Books Online that comes with the install! It's the 1st place I would refer to for ANY SQL Server related topics.

If it's practical "how should I do this?" kind of questions, then StackOverflow would be a better place to ask.

Also, I haven't been back for a while but sqlservercentral.com used to be one of the top SQL Server related sites out there.