What is a Crontab?
A cron is a utility that allows tasks to automatically run in the background of the system at regular intervals by use of the cron daemon. Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at what times they are to be run. This can be quite useful. For example, you may have a personal temporary directory that you wish to be cleaned out once a day to keep your quota from being exceeded. This is where cron scheduling comes in to play. Not all systems allow for a cron schedule to be setup. You need to see your system administrator to see if it is available on your system.

How does it work?
A cron schedule is a simple ASCII text file. Each user has their own cron schedule. This is normally located in /var/spool/cron/crontabs for linux machines. The crontab files are not edited (or created) directly and you do not have access to the file without invoking it from the crontab command. You may not use any text editor you wish. You must use the text editor that has been specified in you system variables (see your system administrator for these). The text editor vi is usually the default text editor. The editor must be invoked using the -e switch. To create a cron schedule type:

crontab -e