Syntax:
TRUNCATE [TABLE] tbl_name

TRUNCATE TABLE empties a table completely. It requires the DROP
privilege as of MySQL 5.1.16. (Before 5.1.16, it requires the DELETE
privilege).

Logically, TRUNCATE TABLE is equivalent to a DELETE statement that
deletes all rows, but there are practical differences under some
circumstances.

For an InnoDB table:

o If there are no FOREIGN KEY constraints, InnoDB performs fast
  truncation by dropping the original table and creating an empty one
  with the same definition, which is much faster than deleting rows one
  by one.

o When you use this fast truncation technique with the
  innodb_file_per_table option enabled, the operating system can reuse
  the freed disk space. For users of the InnoDB Plugin, the space is
  reclaimed automatically, as described in TRUNCATE TABLE Reclaims
  Space
  (http://dev.mysql.com/doc/refman/5.5/en/innodb-other-changes-truncate
  .html). If you do not have the InnoDB Plugin installed, issue the
  OPTIMIZE TABLE statement to free the disk space for the table.

o If there are any FOREIGN KEY constraints that reference the table,
  InnoDB processes TRUNCATE TABLE by deleting rows one by one,
  processing the constraints as it proceeds. If the FOREIGN KEY
  constraint specifies DELETE CASCADE, rows from the child (referenced)
  table are deleted, and the truncated table becomes empty. If the
  FOREIGN KEY constraint does not specify CASCADE, the TRUNCATE TABLE
  statement deletes rows one by one and stops if it encounters a parent
  row that is referenced by the child, returning this error:

ERROR 1451 (23000): Cannot delete or update a parent row: a foreign
key constraint fails (`test`.`child`, CONSTRAINT `child_ibfk_1`
FOREIGN KEY (`parent_id`) REFERENCES `parent` (`id`))

  *Note*: In MySQL 5.5 and higher, TRUNCATE TABLE is not allowed for
  InnoDB tables referenced by foreign keys. For ease of upgrading,
  rewrite such statements to use DELETE instead.

o The AUTO_INCREMENT counter is reset to zero by TRUNCATE TABLE,
  regardless of whether there is a FOREIGN KEY constraint.

This is the same as a DELETE statement with no WHERE clause.

The count of rows affected by TRUNCATE TABLE is accurate only when it
is mapped to a DELETE statement.

For other storage engines, TRUNCATE TABLE differs from DELETE in the
following ways in MySQL 5.1:

o Truncate operations drop and re-create the table, which is much
  faster than deleting rows one by one, particularly for large tables.

o Truncate operations cause an implicit commit.

o Truncation operations cannot be performed if the session holds an
  active table lock.

o Truncation operations do not return a meaningful value for the number
  of deleted rows. The usual result is "0 rows affected," which should
  be interpreted as "no information."

o As long as the table format file tbl_name.frm is valid, the table can
  be re-created as an empty table with TRUNCATE TABLE, even if the data
  or index files have become corrupted.

o The table handler does not remember the last used AUTO_INCREMENT
  value, but starts counting from the beginning. This is true even for
  MyISAM and InnoDB, which normally do not reuse sequence values.

o When used with partitioned tables, TRUNCATE TABLE preserves the
  partitioning; that is, the data and index files are dropped and
  re-created, while the partition definitions (.par) file is
  unaffected.

o Since truncation of a table does not make any use of DELETE, the
  TRUNCATE TABLE statement does not invoke ON DELETE triggers.

URL: http://dev.mysql.com/doc/refman/5.1/en/truncate-table.html

