Understanding Single Sign-On (SSO) for a Newbie

The user understands the basics and benefits of SSO: users log in via a trusted source that authenticates them to other applications, without having to log in repeatedly, while benefiting from the security of centrally managed credentials. They are wondering if SSO is only used for user accounts and not for non-human accounts, and if SSO is only used at the application level, rather than to secure database or operating system accounts.