Single Sign On is when only one service handles authentication of your account, and tells other services either "yes" or "no" based on if you're logged in with them or not.
Popular examples include "sign in with Facebook" or "Sign in with Google". Facebook and Google are the SSO providers in this case. When you click the button to sign in with them, they will tell whatever website that asked whether or not you're able to authenticate your account.
The website asking doesn't need to authenticate. They just have to ask facebook or google if they can verify that you have.
Many people assume this means that they won't have an account with the new website, or that the new website simply "uses" your SSO account. This is false. The new website will create an account for you, but simply moves the burden of authentication to the SSO provider, so the new website won't need to mess around with passwords or security.
Most also think this protects much of their information, but it is quite the contrary. Usually an SSO authentication request with also include a full account permission request, meaning the new website can access all your information from facebook or google.
ASSO, or "Anonymous single sign on", keeps this concept, but doesn't let the new website identify who you are.
HiddenLogin, while not a true SSO provider yet, works upon a similar principle. It uses website's built in authentication, but does not let them identify you. By managing all your logins, it's operationally similar to an ASSO, but requires no development input on websites behalf, and can be universally used everywhere.