Very Computer

I create a job with Operating system commands and TSQL command with successfully result
now I have a question with scheludes tab page in job properties:

    in this page we have "new schedule..." and "new alert..." buttons to create new one for both
of them.
 I tested "new scedules...." with no problem.
  now I want any help to show me a simple test or example with "new alert..." in this page
 I want to know what's its benefits here? and how it fires? and a simple example of it to feel
 better this subject.
I studied helpbook with no example found for alerts in job properties.
 (but I familiar with alerts actions and I tested them out of job operations)

Can anybody help me?
Thanks
Maryam