TUX is also a kernel patch that brings some web server functionality to the linux kernel. It has superceded khttpd which has fallen into disrepair. TUX will likely be included into the mainstream kernel eventually (though there is no sign of it happening currently). TUX is able to serve static pages straight from kernel space, vastly improving performance over userspace deamons like apache. It can cache dynamic content, which will also increase performance. TUX supports virtual hosting. Anything that TUX doesn't understand how to deal with, it passes to a standard httpd, such as apache.
UPDATE: RedHat has apparently filed for some software patents on parts of TUX, I will get more information when I find it