WSS introduces a powerful new extensibility mechanism known as delegate controls. In some ways, a delegate control is similar to a named placeholder because it defines a region inside a master page that can be substituted with unique content to meet the needs of a particular business solution. Like a placeholder, a delegate control can optionally supply default content that is used until a substitution is performed.
One major difference when compared to placeholders is that the substitution mechanism for replacing the contents of a delegate control is driven through feature activation. Therefore, you can replace what’s defined inside a delegate control in default.master without requiring any changes to default.master or the site pages that link to it. All you need to do is define a Control element within a feature and then activate that feature.