The concept of a single sign-on is better explained by Wikipedia:
Single sign-on (SSO) is a property of access control of multiple related, but independent software systems. With this property a user logs in once and gains access to all systems without being prompted to log in again at each of them. Single sign-off is the reverse property whereby a single action of signing out terminates access to multiple software systems.
Have a look at the link below to get a better understanding about the process.
Wiki
Cheers,
Raj.