HTML controls are the native browser elements, such as buttons, textboxes,
tables, etc.
There aren't any "HTML server controls" per se, but there are frameworks
such as ASP.NET that make it seem as though you can inject server-side logic
inside HTML pages by using server-side controls. These pages are not really
HTML pages because they are interpretted by a runtime on the server before
the HTML is sent to the browser. As a result, they usually have different
file extensions (such as .asp or .aspx) so the server knows which files to
interpret (the ASP.NET files) and which to send as-is (the HTML).
ASP.NET controls are executed on the server, with the resultant HTML sent to
the client. ASP.NET controls also emit JavaScript when applicable (such as
for client-side validation) to improve performance. A big benefit to ASP.NET
is that emits standards-based content to a varied array of browsers so
there's no deployment required for end users.
As for performance, the best bet is probably to make any files that have no
dynamic content plain HTML and make all dynamic pages in ASP.NET.
