Site provisioning in MOSS is the process of creating an instance of website/application using the pre-defined site definitions.
While creating an instance, the SP admin will choose a template that are registered in the server farm, (site definitions). In case, the site need to be modified after it has been provisioned,
Features allows change to an existing functionality associated with a provisioned site. Like SiteDefinition, Features is a collection of XML files that, once registered, becomes available to site administrators. A Feature applied at a SPSite scope is automaticallyavailable to all nested SPWebs