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.

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. The
log files also are removed from the list recorded in the index file, so
that the given log file becomes the first.

This statement has no effect if the --log-bin option has not been
enabled.

URL: http://dev.mysql.com/doc/refman/5.1/en/purge-binary-logs.html

