The Truth About Servers Page 3
Web servers grew "brains" by relying on additional technologies so they could process pages before delivering final results to the client. The common gateway interface, or CGI, was the first popular technology that allowed a Web server to interact with an external computer program that could crunch data and deliver the results back to the Web server, before reaching the client. Embedded server-side scripting technologies such as Microsoft's Active Server Pages (ASP) and the open-source PHP begot another evolution, allowing developers to insert their processing code directly into the Web page itself.
The "application server," is an umbrella term that really encompasses all of these skills. Because these servers possess an extensive ability to process information, interaction with the client become like an application with give-and-take between the user and computer, rather than like a book, where the reader is limited to flipping through pages.
The achievements of an application server can certainly be enjoyed with an intelligent combination of existing technologies. For example, an open source developer might connect an Apache Web server with the PHP scripting language. In effect, this is an application server. But as we see the term used in the marketplace, an "application server" is typically a bundled solution offered by a vendor that contains all the component technologies needed. For some organizations, the bundled approach of a self-proclaimed application server eases development by unifying development models and centralizing support.