Single sign-on (SSO) means that one user ID and one password are entered one time to allow passage from one system to another without interruption. In the realm of web development, this means you can log in on one site and then use the same credentials while moving from one domain to the next. This gets interesting because neither cookies nor JavaScript are allowed to directly share such information across domains. This means that the web applications must perform a little extra footwork to get this communication done. You also need to be concerned that you aren't giving too much information away too easily, so it's also important to allow the user to have a say in the matter during these transactions.
