Clustered and nonclustered indexes share many of the same internal structures, but they're fundamentally different in nature. In this video, I compare the similarities and differences of clustered and nonclustered indexes, using a real-world example to show how these structures work to improve the performance of SQL queries.
Blog post on primary key vs. the clustered index
CREATE INDEX statement reference
ALTER INDEX statement reference
Index navigation internals by example
Sample index data is from the AdventureWorksLT2008R2 sample database