<p>I understand SSO as the one time login for all the applications connected. I would like to know more about SSO and how exactly it is achieved.</p>

This is Donnie. He's a friend of mine.
The basic mechanism is to trust your friend's friend.
Another metaphor that's used is the sticker they give you when you enter a secured building.  In the morning, you show your valid ID (credential) to the security person, and for the rest of the day, the sticker acts as the pass (token). When you come back next day, you have to show your ID again.
Key technology is how you trust your friend or security guy or anyone in hostile network. See how Kerberos or asymmetric encryption work.