Good Additions. I would also point out that if devs mess with indexes they definitely need to know the difference between clustered and nonclustered. I've seen atrocities due to a simple misunderstanding of this (guess what happens when you confuse the two and implement the opposite!)
Clustered is directly related to the physical order of the records. Non-clustered is a logical order that uses pointers to the physical location. Only one clustered is allowed per table.
The concept also does not exist in some databases. Oracle, IIRC, does not have the notion of a clustered index. SQL Server does. Another illustration of why you should not assume that all relational databases work the same way.
