This is an excellent read for a beginner just learning about indexing. Here is an excerpt from the article on SQLTeam.Com
Indexes directly affect the performance of database applications. This article uses analogies to describe how indexes work. The estimated execution plan feature of the Query Window is utilized to compare the performance of two queries in a batch.
Most developers and DBAs understand that indexes on database tables enable applications and reports to run more efficiently. But, do they really know whether the database system will take advantage of an index to process a particular query or update statement? Does the order of columns specified in the index matter? What is the difference between a clustered index and a non-clustered index? To understand some index concepts, I have found it useful to use some analogies.