JBoss WS is the implementation of JAX-WS standard which is provided together with JBoss AS. JBoss WS is developed by Red Hat, so it is "native" implementation for JBoss (for JBoss 5.X and JBoss 6.x series).
So yes, you need JBossWS for web services to work. But...
But in case of JBoss 5.X.GA JBoss WS can be replaced by Apache CXF or Metro implementations of JAX-WS. Anyway, you need some JAX-WS implementation.
In case of the newest JBoss version - 7.X, JBossWS is abandoned and Apache CXF is used.
There is also jbossws Web application (available on local machine at localhost:8080/jbossws) - this application is not needed for webservices to work, but it is convienient for downloading WSDL files or checking if web service was deployed - it can be password protected, if needed.