JBoss is an implementation of the EJB 1.1 (and parts of 2.0) specification,
that is, it is a server and container for Enterprise JavaBeans. In this it
is similar to Sun's 'J2SDK Enterprise Edition' (J2EE), but the JBoss core
server provides only an EJB server. The JBoss core does not include a web
container for servlets/JSP pages, although there are bundles available that
include either Tomcat or Jetty. The minimal core offering means that JBoss
has minimal memory and disk space requirements. JBoss will run very
effectively on a machine with 64 megabytes of RAM, and requires only a few
megabytes of disk (including source code!). Sun's J2EE requires a minimum of
128 megabytes of RAM, and 31 megabytes of disk space. Because of its small
memory footprint, JBoss starts up about 10 times faster than J2EE. There is
a built-in SQL database server for handling persistent beans, and this
starts up automatically with the server (J2EE ships with the CloudScape SQL
server, which has to be started separately).

One of the nicest features of JBoss is its support for `hot' deployment. What
this means is that deploying a Bean is a simple as copying its JAR file into
the deployment directory. If this is done while the Bean is already loaded,
JBoss automatically unloads it, then loads the new version. Contrast this
with the rigmarole that other J2EE server makes us go through... JBoss is
distributed under the LGPL, which means that it's free, even for commercial
work, and the LGPL ensures that it remains that way. 

WWW: http://www.jboss.org/