Hub is a Ruby On Rails application with accompanying library gem which manages user accounts for a web site. Its main use is for sites that run more than one Rails application within a single domain. Through integration with the Hub library gem, applications share Hub user details so that a user only has to create one account and log in or out of one place. Without Hub, users have to create and manage individual accounts for individual applications. Hub is therefore a single sign-on mechanism.
For more information, please see the Hub project page. The ROOL Wiki page you’re reading now describes Hub customisations carried out for this web site.