I forgot to mention in my previous post an important aspect of the Web Part Framework.  There is the concept of Web Part Zone controls that allow you to set up containers for Web Part controls or other specialized management features.  Specifically, there is a WebPartZone container class used for Web Part controls, an EditorZone used to change properties on selected Web Part control instances, a CatalogZone used to add new Web Parts to a page, and a ConnectionsZone that can be used to manage connections between Web Parts.

There are five Web Part display modes: Browse, Design, Catalog, Editor, and Connections.  Under Design mode, for example, users can drag and drop (IE) web parts around on the page.  Under Catalog display mode, the CatalogZone container is visible and users can choose from a list of available Web Part controls to add to one of the available WebPartZone containers on the page.

Web Parts Control Set Overview on MSDN