Syntax:
OPTIMIZE [NO_WRITE_TO_BINLOG | LOCAL] TABLE
    tbl_name [, tbl_name] ...

OPTIMIZE TABLE should be used if you have deleted a large part of a
table or if you have made many changes to a table with variable-length
rows (tables that have VARCHAR, VARBINARY, BLOB, or TEXT columns).
Deleted rows are maintained in a linked list and subsequent INSERT
operations reuse old row positions. You can use OPTIMIZE TABLE to
reclaim the unused space and to defragment the data file. After
extensive changes to a table, this statement may also improve
performance of statements that use the table, sometimes significantly.

This statement requires SELECT and INSERT privileges for the table.

URL: http://dev.mysql.com/doc/refman/5.0/en/optimize-table.html

