The Connection Server is the name of this site. That's right, there is no technology officially called a 'connection server'. However it could describe any number of technologies that we use everyday in the corporate sys admin world. Any type of server must accept connections from clients in order to respond, so whether its a HTTP server, JEE server or sql/database server, its pretty much a connection server too. The articles at this site will touch on all these types of technologies.
In case you were wondering...
HTTP Servers, which handle HTTP connections, accept and process requestsand return HTML code which is processed by an internet browser (Eg. Internet Explorer, Firefox, Chrome, Safari and Opera). The internet browser renders the HTML code as formatted text and graphics. Common HTTP servers include Apache and IIS.
Application Servers, such as WebLogic, WebSphere, JBoss and Tomcat, usually include a basic HTTP server, but are also able to run JEE (Java Enterprise Edition) applications which typically perform some business logic/rules and respond with HTML code to the internet browser.
Database Servers, accept SQL connections and process SQL queries to manipulate or return (to an application) data in database tables. Common database servers include Oracle, Microsoft SQL Server, MySQL, PostgreSQL and Pointbase.