-A Web Form refers to the .aspx page that inherits from the System.Web.UI.Page class.
-There is also an HtmlForm class that allows you to create and control an Html <Form> tag on the server. You can create this in your .aspx page using <form runat="server"> and must place your input related tags/controls (buttons, text boxes, etc.) inside it. This will be rendered to the client as the standard HTML <form> tag.
-Web Controls are a group of classes in the System.Web.UI.WebControls namespace, such as labels, repeaters and datagrids, as well as the form related controls such as text boxes and buttons.
-Html Controls are a group of classes similar to the web controls but they are in the System.Web.UI.HtmlControls namespace and are very simple controls that represent Html tags such as the HTML form mentioned above, buttons, text boxes, etc. HtmlControls generally are not as feature rich as Web Controls and there are less of them.