cron is a built-in utility in Linux that makes program/application/process run without manual intervention.
You can use following to start,stop,restart,and reload the service in your system:

 /sbin/service crond start 

  /sbin/service crond stop 

  /sbin/service crond restart 

  /sbin/service crond reload

And configure scheduled tasks in this format:

minute hour day  month week [user_name] command

minute: means which minute in hours to run the task,range from 0-59.
hour:means which hour to run the task,range from 0-23.
day:means which day to run the task,range from 1-31.
month:means which month to run the task,range from 1-12.
week:means which day in one week to run the task,range from 0-6.0 stands for Sun.
user_name: means use which user to run the task,leave out to use current user.
command: command and parameter to run the task.

In the directory /etc,there are several folders such as cron.daily,cron.hourly,cron.monthly,cron.weekly.You can create shell into one of these folders.
For example,we want nutch run at every 9:00 am:

cd /etc/cron.daily
chmod 755
echo "/home/sunny/nutch/bin/nutch crawl urls -dir crawl" >

For desktop version,there’s a GUI available from Applications/Add&Remove named Scheduled tasks.