A Salesforce user is anyone who logs in, and the Username field is the login identity: it must look like an email address and must be unique across every Salesforce org, including trials and sandboxes. Salesforce Identity then layers on how that person proves who they are, such as password plus required multi-factor authentication, single sign-on through a Federation ID, social sign-on through an authentication provider, or password-free Lightning Login. Administrators manage those methods on the user record and in Identity settings so people can reach Salesforce and connected apps from one place without a separate password for every system.