Apache Tomcat is an open source software implementation of the Java Servlet and JavaServer Pages technologies. The Java Servlet and JavaServer Pages specifications are developed under the Java Community Process.
Apache Tomcat is developed in an open and participatory environment and released under the Apache License version 2. Apache Tomcat is intended to be a collaboration of the best-of-breed developers from around the world. We invite you to participate in this open development project. To learn more about getting involved, click here.
Apache Tomcat powers numerous large-scale, mission-critical web applications across a diverse range of industries and organizations. Some of these users and their stories are listed on the PoweredBy wiki page.
The most well known and most frequently used servlet container. Works pretty well for what it does (a pure servlet container), but few, very few people would use this as-is. As a rule of thumb people add tons of libraries to Tomcat (like JSF, JPA, CDI, etc). At some you'll be better off installing a readily made full stack, like the Tomcat based TomEE.
One thing that I really dislike about Tomcat is that it copies the context.xml from a war when it first encounters a particular war and stores it internally. If you deploy a new war with a new context.xml, Tomcat will continue using the old one. The Tomcat developers are a HUGE fan of this feature, but I just don't see the point. Why would one ever want to keep the first version of context.xml encountered and silently ignore newer versions?
Nevertheless, this is -the- hallmark for Java based servers, and every Java professional should have Tomcat experience under his belt.