Single Sign-On is a heavily used term in the IT industry. Sometimes inaccurately used to refer to any tool that simplifies login for the end-user.
The 3 most commonly used forms on sign-on are:
Single Sign-On: The user is required to log-in only once on Active Directory or another directory service. After logging in, the user is prompted to enter credentials to access apps/resources that integrate with the directory service.
Single Log On (aka centralised authentication): The user logs in on Active Directory or another directory service and all apps/resources using the same credentials. The user will see log-in dialogs for each individual app or resource requiring these credentials.
Social Login: A form of single sign-on using existing information from a social networking or SaaS service (such as LinkedIn, Facebook, Microsoft services) to sign into a 3rd party website/app instead of creating a new set of credentials specifically for that website/app. It is designed to simplify logins for end users as well as provide more and more reliable demographic information to web application developers.
It is important that the distinction between the different forms of sign-on are understood as they all have different use cases, and levels of security.