Event handlers or to say ITEM based event  Handlers are used to control the events happening on the item (basically the entire row i.e. all the fields  in the row by which you can control commiting of the changes. So this way you can perform validation and depending on the results of validation you can commit or rollback the changes.

Check this article: blogs.msdn.com/.../part-1-event-

handlers-everything-you-need-to-know-about-microsoft-office-sharepoint-portal-server-moss-

event-handlers.aspx