ADN -- (Advanced Digital Network)
Usually refers to a 56Kbps leased-line.

ADSL -- (Asymmetric Digital Subscriber Line)
A DSL line where the upload speed is different from the download speed. Usually the download speed is much greater.

Ajax -- (Asynchronous JavaScript and XML)
A way of including content in a web page in which javascript code in the web page fetches some data from a server and displays it without re-fetching the entire surrounding page at the same time (hence the 'Asynchronous')

Often (but not always) the data fetched by the javascript code is in XML format.
It is common for Ajax applications to update the Ajax content multiple times without the surrounding page needing to be updated even once.

A simple example of Ajax would be a weather-forcast box in the middle of a web page. Ajax could be used to populate the box every 5 minutes without needing to refresh the surrounding page.

Apache
The most common web server (or HTTP server) software on the Internet. Apache is an open-source application originally created from a series of changes ("patches") made to a web server written at the National Center for Supercomputing Applications, the same place the Mosaic web browser was created.

Apache is designed as a set of modules, enabling administrators to choose which features they wish to use and making it easy to add features to meet specific needs inlcuding handling protocols other than the web-standard HTTP.

Applet
A small Java program that can be embedded in an HTML page. Applets differ from full-fledged Java applications in that they are not allowed to access certain resources on the local computer, such as files and serial devices (modems, printers, etc.), and are prohibited from communicating with most other computers across a network. The common rule is that an applet can only make an Internet connection to the computer from which the applet was sent.

Application Server
Server software that manages one or more other pieces of software in a way that makes the managed software available over a network, usually to a Web server. By having a piece of software manage other software packages it is possible to use resources like memory and database access more efficiently than if each of the managed packages responded directly to requests.

Archie
A tool (software) for finding files stored on anonymous FTP sites. You need to know the exact file name or a substring of it. By 1999 Archie had been almost completely replaced by web-based search engines.

Back when FTP was the main way people moved files over the Internet archie was quite popular.

ARPANet -- (Advanced Research Projects Agency Network)
The precursor to the Internet. Developed in the late 60's and early 70's by the US Department of Defense as an experiment in wide-area-networking to connect together computers that were each running different system so that people at one location could use computing resources from another location.

ASCII -- (American Standard Code for Information Interchange)
This is the defacto world-wide standard for the code numbers used by computers to represent all the upper and lower-case Latin letters, numbers, punctuation, etc. There are 128 standard ASCII codes each of which can be represented by a 7 digit binary number: 0000000 through 1111111.

ASP -- (Application Service Provider)
A organization (usually a business) that runs one or more applications on their own servers and provides (usually for a fee) access to others. Common examples of services provided this way include web-based software such as Calendar systems, Human Resources tools (timesheets, benefits, etc.), and various applications to help groups collaborate on projects.

Atom
An evolving protocol for syndication and sharing of content.

Atom is being developed as a succesor to and improvement over RSS and is more complex than RSS while offering support for additional features such digital signatures, geographic location of author, possibly security/encryption, licensing, etc.

Like RSS, Atom is an XML-based specification.