Single sign-on is a property of login to multiple resources within an organization using the same username and password. With this technology, a user will get access to all resources by login once without being prompted to login again and again. The process is generally accomplished using the LDAP( Lightweight Directory Access Protocol).
The term is a little bit confusing. There are two cases; first is: users only needs to enter credentials one time per session and get access to login to multiple web resources without having sign in again during that session. Second is: users can access multiple resources using the same credentials but they have to login to each resource individually. Most of the people consider first case as true SSO.
No matter, which one, the main reason behind why businesses are widely implementing SSO solution is because it improves user experience and acceptance. It also saves business money and improves security. This is the basic concept behind web single sign-on solution. One key = many websites and resources.
Now, what is reduced sign-on? The concept of reduced single sign-on is quite similar to SSO. Like SSO, a user can login to many websites and resources using one set of credentials. In this method, businesses can protect some particular websites by adding an additional layer of security. Suppose you have a business and have multiple websites. On some of the websites users can login easily by simply using their credentials. But at the same time there are many higher risk websites and applications like payroll systems where SSO system requires them to use a strong authentication. Here, along with username and password there is an additional layer of security. This includes some certificates, smart cards, biometrics, security tokens, otp etc. After verifying themselves they get the permission to access such resources.
So, we can simply say that single sign-on has transformed into reduced sign-on. The process is safer than SSO.