Syntax:
PURGE { BINARY | MASTER } LOGS
    { TO 'log_name' | BEFORE datetime_expr }

The binary log is a set of files that contain information about data
modifications made by the MySQL server. The log consists of a set of
binary log files, plus an index file (see
http://dev.mysql.com/doc/refman/5.5/en/binary-log.html).

The PURGE BINARY LOGS statement deletes all the binary log files listed
in the log index file prior to the specified log file name or date.
BINARY and MASTER are synonyms. Deleted log files also are removed from
the list recorded in the index file, so that the given log file becomes
the first in the list.

This statement has no effect if the server was not started with the
--log-bin option to enable binary logging.

URL: http://dev.mysql.com/doc/refman/5.5/en/purge-binary-logs.html

