It is hard to give a short answer to your question as indexes are the core of sql server databases. You can srat with BOL, also look on internet for specific details. In short, a non-clustered index makes sense only if a clustered index exists: in a table that has a clustered index, the data is actually stored in the order of the index. What the bookmarks of the non-clustered index (read in BOL for definition) point to depends on if the table also has a clustered index or not.