The userlib directory is used by BlueJ as a repository of class 
libraries (as either jar or zip files).  Class libraries in this
directory are recognised and added to BlueJ's classpath at 
start-up.

Loaded libraries can be viewed via the User Library List displayed in 
the Preferences dialog accessible via the Tools>Preferences...>Libraries 
menu option.  Libraries placed here will be accessible by all BlueJ projects 
and all users.