Typically, a combination of a user name and a corresponding password will be used, since this allows access to be issued and revoked (as well as audit logging) on an individual basis, making a single person responsible for use and abuse of the password.
In case of confidential information, passwords will be required even for read-only access, with different users having access to different subsets of information. But even for public information, passwords are often used to selectively grant edit privileges. Probably the most common use of passwords is to limit access to user account information to the owner of the account.
Secure storage of passwords is an important topic, neglect of which can lead to significant security breaches, especially due to password reuse. Use of secure password hashes or, even better, purpose-built algorithms such as BCrypt for this purpose is critical.