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RADIUS, which stands for Remote Authentication Dial-In User Service, is specifically designed for centralized authentication, authorization, and accounting (often referred to as AAA) for users accessing network services. It operates primarily in network environments where remote access servers need a reliable method to manage user logins and to enforce policies across the network.

RADIUS functions by allowing the remote server to communicate with a centralized server to verify the credentials of users trying to gain access. This ensures that all users are properly authenticated and authorized based on their credentials and predefined policies, while also keeping logs of user activity, which is essential for security audits and reporting.

The other protocols mentioned serve different purposes: LDAP (Lightweight Directory Access Protocol) is primarily used for accessing and maintaining directory information, which can play a role in authentication but is not specifically tailored for accounting. SAML (Security Assertion Markup Language) is used for single sign-on and exchanging authentication and authorization data between parties, and SSH (Secure Shell) is used for secure access to network devices and servers, but does not provide centralized AAA functionality like RADIUS does.