Syntax:
TRUNCATE [TABLE] tbl_name

TRUNCATE TABLE empties a table completely. It requires the DROP
privilege.

Logically, TRUNCATE TABLE is similar to a DELETE statement that deletes
all rows, or a sequence of DROP TABLE and CREATE TABLE statements. To
achieve high performance, it bypasses the DML method of deleting data.
Thus, it cannot be rolled back, it does not cause ON DELETE triggers to
fire, and it cannot be performed for InnoDB tables with parent-child
foreign key relationships.

Although TRUNCATE TABLE is similar to DELETE, it is classified as a DDL
statement rather than a DML statement. It differs from DELETE in the
following ways in MySQL 5.5:

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, and so cannot be rolled
  back.

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

o TRUNCATE TABLE fails for an InnoDB table if there are any FOREIGN KEY
  constraints from other tables that reference the table. Foreign key
  constraints between columns of the same table are allowed.

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 Any AUTO_INCREMENT value is reset to its start value. 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 The TRUNCATE TABLE statement does not invoke ON DELETE triggers.

URL: http://dev.mysql.com/doc/refman/5.5/en/truncate-table.html

