Application servers are java programs that serve active content to web browsers. Active content is information that changes over time. The balance in your checkbook is an example of active content. It changes every time you write a check or make a deposit. Stock and bond prices, inventory levels, the number of people in a pub at any given moment, the position of an airplane as it flies to its destination -- these are all examples of active content. If you want to publish this kind of information on a web site, you have to do more than write plain HTML. You have to write a computer program that
- knows where the information you need can be found
- finds the information you need
- generates the markup language -- typically HTML -- that will present your information to an end user
- sends that markup text to a web browser
That computer program might be a servlet, an Enterprise Java Bean, some plain old Java code, or some combination of the above. An application server runs that kind of program. There are several application servers. Some of he more popular ones are:
All of those application servers are fine products. Each of them serves a need. Most of them are meant to either stand alone or cluster in small groups of servers. One of them -- WebSphere Application Server -- is, instead, a network of potentially thousands of servers all providing pieces of a very large web site.