Spring Security began as a third-party security module called Acegi Security and was later included as an official part of the Spring portfolio. It provides authentication, authorization and protection for Spring-based applications and includes modules for core security, web security, configuration, LDAP integration, access control lists, CAS support and OpenID integration. The document discusses what Spring Security is, why it is needed, the difference between authentication and authorization, session management, LDAP authentication, and shows how Spring Security acts as a filter to secure applications.