What is SSO (Single Sign-On)?

A friend recently went through the irksome experience of being signed out from a number of websites they use daily. This event will be familiar to millions of web users, and it is a tedious process to fix. It can involve trying to remember multiple long-forgotten passwords, or typing in the names of pets from childhood to answer security questions. SSO removes this inconvenience and makes life online better. But how does it work? Basically, Single Sign-On (SSO) is an authentication scheme. It allows a user to log in to different systems using a single ID. The diagram below illustrates how SSO works.