Spring Security is a framework for authentication and authorization in Java applications. It provides components for authentication filters, providers, and managers as well as user details services. Basic authentication uses HTTP basic auth with a username and password encoded in the request header. Form authentication displays a login form and uses CSRF protection. Custom authentication allows multiple auth types by implementing custom auth providers, users, grants, and handlers. The documentation covers getting started, architecture, basic auth, form auth, and custom auth configurations and demos.