Feature Stapling

One of the big problems people faced in WSS (V2) was that user was not able to customize out of the box site definitions. One of the big reasons for this was that we might need to update that site definition in a Service Pack and overwrite user changes.
So what happens if you want to associate a Feature with a site definition?
Well, as you should know by now, it is not advisable to modify the out of the box site definitions. So how do you do this?
This is where Feature Stapling comes in...

Feature Stapling allows you to “staple” a Feature to a site definition without modifying it in any way. This means you can add your feature to all sites created using that site definition.
The idea is that you can staple a Feature to a configuration defined within a site definition so that the Feature is automatically activated whenever a new site is provisioned from that specific configuration.
To create a staple you actually create another Feature that does the staple. The second Feature must provide a FeatureSiteTemplateAssociation element which specifies the identifying GUID of the target feature along with a formatted string used to identify a site definition and a target configuration.
for example:

<FeatureSiteTemplateAssociation Id="29D85C25-170C-4df9-A641-12DB0B9D4130" TemplateName="STS#0" />

This is very powerful as it allows you to add functionality to site definitions without having to modify the site definitions themselves.

Now ... one last final note on this, if you want to staple your Feature to ALL site definitions then you can staple it to the GLOBAL site definition and it will be added to all sites that are created.