What's clustered index of sql server

Keeping the Data in Order
As previously stated, a clustered index actually keeps the data in the table in a specific order. When
you specify a column (or multiple columns) as a clustered index, on inserting a record SSE will
place that record in a physical position to keep the records in the correct ascending or descending
order that corresponds to the order defined in the index. To explain this a bit further, if you have a
clustered index on customer numbers, and the data currently has customer numbers 10, 6, 4, 7, 2,
and 5, then SSE will physically store the data in the following order: 2, 4, 5, 6, 7, 10. If a process then
adds in a customer number 9, it will be physically inserted between 7 and 10, which may mean that
the record for customer number 10 needs to move physically. Therefore, if you have defined a clustered
index on a column or a set of columns where data insertions cause the clustered index to be
reordered, this is going to greatly affect your insert performance. SSE does provide a way to reduce
the reordering impact by allowing a fill factor to be specified when an index is created.