Web Forms Application Namespaces

System.Web.UI	Classes and interfaces for creating user interface pages and controls in web applications. Includes the Control base class, the Page class and classes to implement data binding, viewstate management, and control parsing.
System.Web.UI. Design	Classes for extending design-time support for Web Forms.
System.Web.UI. Design.WebControls	Classes for extending design-time support for Web Controls.
System.Web.UI. HtmlControls	Classes for creating HTML server controls that map directly to standard HTML elements.
System.Web.UI. WebControls	Classes for creating ASP.NET Web Controls, which provide a consistent and abstracted interface.