Syntax:
Single-table syntax:

UPDATE [LOW_PRIORITY] [IGNORE] tbl_name
    SET col_name1=expr1 [, col_name2=expr2 ...]
    [WHERE where_condition]
    [ORDER BY ...]
    [LIMIT row_count]

Multiple-table syntax:

UPDATE [LOW_PRIORITY] [IGNORE] table_references
    SET col_name1=expr1 [, col_name2=expr2 ...]
    [WHERE where_condition]

For the single-table syntax, the UPDATE statement updates columns of
existing rows in tbl_name with new values. The SET clause indicates
which columns to modify and the values they should be given. The WHERE
clause, if given, specifies the conditions that identify which rows to
update. With no WHERE clause, all rows are updated. If the ORDER BY
clause is specified, the rows are updated in the order that is
specified. The LIMIT clause places a limit on the number of rows that
can be updated.

For the multiple-table syntax, UPDATE updates rows in each table named
in table_references that satisfy the conditions. In this case, ORDER BY
and LIMIT cannot be used.

where_condition is an expression that evaluates to true for each row to
be updated. It is specified as described in [HELP SELECT].

The UPDATE statement supports the following modifiers:

o If you use the LOW_PRIORITY keyword, execution of the UPDATE is
  delayed until no other clients are reading from the table. This
  affects only storage engines that use only table-level locking
  (MyISAM, MEMORY, MERGE).

o If you use the IGNORE keyword, the update statement does not abort
  even if errors occur during the update. Rows for which duplicate-key
  conflicts occur are not updated. Rows for which columns are updated
  to values that would cause data conversion errors are updated to the
  closest valid values instead.

URL: http://dev.mysql.com/doc/refman/5.0/en/update.html

