Some more index information!
Clustered indexes are the actual physical layout of the records in the table. Hence, you can only have one per table.
Nonclustered indexes are the aforementioned card catalog. Sure, the books are arranged in a particular order, but you can arrange the cards in the catalog by book size, or maybe by number of pages, or by alphabetical last name.
Something to think about -- creating too many indexes is a common pitfall. Every time your data gets updated your DB has to seek through that index and update it, inserting a record into every index on that table for that new row. In transactional systems (think: NYSE's stock transactions!) that could be an application killer.