What does this sentence mean: Clustered indexes are stored physically on the table?

Question

How are clustered indexes stored on a hard disk? What is the logical order?

How do non-clustered indexes work?

Answer 1

It means that a clustered index determined the physical order in which records in a table are actually stored. Non-clustered indices are simply lists of key values stored separately that enable fast lookups in other orderings than the clustered/physical ordering.

Quick example: a table with ID (primary key), FirstName, LastName and Car containing three people: 0=The Stig (Llana), 1=Jeremy Clarkson (DB9), 2=Richard Hammond (911), 3=James May (Lambo) and a clustered index on LastName and a non-clustered index on Car would store the actual lines of data in the table in this physical order on disk:

ID FirstName LastName Car
1  Jeremy    Clarkson DB9
2  Richard   Hammond  911
3  James     May      Lambo
0  The       Stig     Llana

The nonclustered index would also store something like:

Car   ID
911   2
DB9   1
Lambo 3
Llana 0