some official definitions:
What is the Apache HTTP Server?
* is a powerful, flexible, HTTP/1.1 compliant web server
* implements the latest protocols, including HTTP/1.1 (RFC2616)
* is highly configurable and extensible with third-party modules
* can be customised by writing 'modules' using the Apache module API
* provides full source code and comes with an unrestrictive license
* runs on Windows 2003/XP/2000/NT/9x, Netware 5.x and above, OS/2, and most versions of Unix, as well as several other operating systems
What is the ApacheTomcat? ApacheTomcat is the servlet container that is used in the official Reference Implementation for the Java Servlet and JavaServer Pages technologies. The Java Servlet and JavaServer Pages specifications are developed by Sun under the Java Community Process. ApacheTomcat is developed in an open and participatory environment and released under the Apache Software License.