How do I add jobs to cron under Linux or UNIX oses?

Cron job are used to schedule commands to be executed periodically i.e. to setup commands which will repeatedly run at a set time, you can use the cron jobs.
crontab is the command used to install, deinstall or list the tables used to drive the cron daemon in Vixie Cron. Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be