bin contains run.sh, shutdown.sh, and bootstrapping jar files.
client contains jar files that must be specified when compiling J2EE applications. Note that servlet.jar is in server/default/lib. The server does not use this directory at run time.
docs contains DTDs for deployment descriptors as well as data source configuration scripts for supported RDBMSs.
lib contains jar files needed to by JBoss. Do not put user jar files into this directory.
server contains the "factory" configuration sets. New configuration sets can be created here, starting with a copy of one of the preconfigured sets.
For each configuration set, conf contains configuration files, deploy contain deployed system resources and hot-deployable user applications, and lib contain non-hot-deployable jar files used by the configuration set.
The data, log, and tmp directories are created if necessary when the server is started. The log/server.log file contains lots of useful information.