Feature Stapling in SharePoint 2010

http://msdn.microsoft.com/en-us/library/bb861862(v=office.12).aspx
http://gallery.technet.microsoft.com/office/Feature-Stapling-in-fcbaf891 https://www.nothingbutsharepoint.com/sites/devwiki/articles/Pages/SharePoint-Feature-Stapling-Staple-the-Feature.aspx
This Post Introduction to what is feature stapling in SharePoint 2010 as well as How to use it??

Why we use Feature Stapling?

Features Stapling Uses providing a great way for developers to extend, change, or customize the OOTB site definitions without ever modified them. I means without touching them.

This allows for a feature to be automatically activated on a site created from a site definition without the need to modify the definition files (onet.xml). This also means that:

You can use stapling to attach features to OOTB definitions,

You can use it to "modify" a custom definition that has already been deployed and it's in use.