Since you're asking here, I will assume you mean sites built with openacs.
First, there is the web server itself, which is usually aolserver, which was designed from the start for fast web apps. It has a built in scripting language, TCL, which is used for aolserver config files and for application scripting.
Openacs and apps built using it use a database. There are two supported databases, postgres and oracle.
Finally, openacs itself is a collection of tcl scripts, database scripts and a plugin infrastructuire which allows packages and their development to happen independently of other work; you can write packages which are applications and packages which are support of other packages.
These packages can have user interface (which is normally templated html), a data model which describes how data for the package is stored and how it relates to the rest of the openacs data nodel, a library containing TCL procedure definitions and a catalog of internationalized/localized phrases and sentences.
For more info, see http://openacs.org/