Example: the following will clear the Apache error log at one minute past midnight ( 00:01 of every day of the month, of every day of the week ).

1 0 * * *  echo -n "" > /www/apache/logs/error_log

Slash example: the following will run the script /home/user/test.pl every 5 minutes.

*/5 * * * *  /home/user/test.pl

Cron in Details.

.---------------- minute (0 - 59) 
|  .------------- hour (0 - 23)
|  |  .---------- day of month (1 - 31)
|  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ... 
|  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7)  OR sun,mon,tue,wed,thu,fri,sat 
|  |  |  |  |
*  *  *  *  *  command to be executed