an index has 2 sections
the leaf and non leaf levels.
the leaf level contains indexpages that
In a non-clustered index contain pointers to the actual Data pages for the table
In a Clustered index contain the actual data or to sum it up simply the leaf level of a clustered index is the actual table itself.
the non-leaf level of the index is an organized collection of index pages that are designed to iterate to the proper location in the leaf level.
another way to look at it is that the non-clustered index is just like the index at the back of the book
the index entries are sorted in ascending order but the book itself is not.
a clustered index is analogous to an encyclopedia. the entire collection
of data is sorted in alphabetical order and there is no linking to the data because the data is in the ordered format.
I could go all night but my typing sucks so try looking these up in books online
Using Nonclustered Indexes
Using Clustered Indexes