The software utility Cron is a time-based job scheduler in Unix-like computer operating systems. People who set up and maintain software environments use cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals. It typically automates system maintenance or administration—though its general-purpose nature makes it useful for things like connecting to the Internet and downloading email at regular intervals

Cron is the name of program that enables unix users to execute commands or scripts (groups of commands) automatically at a specified time/date. It is normally used for sys admin commands, like makewhatis, which builds a search database for the man -k command, or for running a backup script, but can be used for anything. A common use for it today is connecting to
the internet and downloading your email.

How to start Cron

Cron is a daemon, which means that it only needs to be started once, and will lay dormant until it is required. A Web server is a daemon, it stays dormant until it gets asked for a web page. The cron daemon, or crond, stays dormant until a time specified in one of the config files, or crontabs.

Cron permissions
The two files play an important role:

/etc/cron.allow – If this file exists, it must contain your username for you to use cron jobs.
/etc/cron.deny – If the cron.allow file does not exist but the /etc/cron.deny file does exist then, to use cron jobs, you must not be listed in the /etc/cron.deny file.
Note that if neither of these files exist then, depending on site-dependent configuration parameters, either only the super user can use cron jobs, or all users can use cron jobs.

Timezone handling

Most cron implementations simply interpret crontab entries in the system time zone setting that the cron daemon runs under. This can be a source of dispute if a large multiuser machine has users in several time zones, especially if the system default timezone includes the potentially confusing DST. Thus, a cron implementation may as a special-case any “CRON_TZ=<timezone>” environment variable setting lines in user crontabs, interpreting subsequent crontab entries relative to that timezone.

Refernce
Wikipedia.com