http://msdn2.microsoft.com/en-us/library/system.web.ui.htmlcontrols.aspx
HTML server controls are HTML elements exposed to the server so you can program against them. HTML server controls expose an object model that maps very closely to the HTML elements that they render.
http://msdn2.microsoft.com/en-us/library/system.web.ui.webcontrols.aspx
Web server controls are ASP.NET server controls with an abstract, strongly-typed object model. Web server controls include not only form-type controls such as buttons and text boxes, but also special-purpose controls such as a calendar. Web server controls are more abstract than HTML server controls, in that their object model does not necessarily reflect HTML syntax.