Web Servers vs. App Servers

These are two quite different pieces of software.
There are few Web servers in common use: Apache takes the lion's share, while Microsoft Internet Information Server (IIS) and iPlanet Web Server are among the others. Application servers jumped into the limelight only about two years ago and there is a proliferation of products from companies such as BEA, iPlanet, Oracle, SilverStream, HP (Bluestone), and IBM.

Speaking of functionality, in differentiating the two servers we have to say "in general" because there are customized versions of both Web and application servers that have crossover functionality. The Web server, in general, sends Web pages to browsers as its primary function. Most Web servers also process input from users, format data, provide security, and perform other tasks. The majority of a Web server's work is to execute HTML or scripting such as Perl, JavaScript, or VBScript.

In general, an application server prepares material for the Web server -- for example, gathering data from databases, applying business rules, processing security clearances, or storing the state of a user's session. In some respects the term application server is misleading since the functionality isn't limited to applications. Its role is more as an aggregator and manager for data and processes used by anything running on a Web server.