Mainly that the pointer to the rest of the row is the clustered index key (if the base table is a clustered index). Not two separate things.

Also, indexes (nonclustered) don't contain data rows, they contain index rows. Only the clustered index has data pages as the leaf level.

http://www.sqlservercentral.com/articles/Indexing/68439/

http://www.sqlservercentral.com/articles/Indexing/68563/

http://www.sqlservercentral.com/articles/Indexing/68636/