<add tagType="Microsoft.SharePoint.WebControls.PeopleEditor" mappedTagType="MyCustomSolutions.Custom.PeopleEditor" />
The above tag if added to web.config , will replace the functionality of the People picker control but the User Interface remains the same.
The tag mapping functionality is a mechanism to instruct the parser to substitute a different derived type implementation whenever it encounters the type being mapped.
It was invented as part of the WebPart framework so as to allow the ASP.NET WebPartManager to be mapped to the derived Sharepoint WebPartManager implementation without having to change individual pages.
This can be used to customize sharepoint people picker with the customized code.