Authentication in WSS 3.0 is configured at the SharePoint Web application level. WSS 3.0 provides flexible authentications. WSS 3.0 supports authentications like Windows authentication methods, forms and Web SSO
Configuring the Windows authentication is straight forward. Eg's of it are Anonymous, basic,digest,Certificates, Kerberos and NTLM.
Configuring forms authentication requires ASP.NET membership providers to be registered in the web.config file. In addition to the registering the membership provider you can also register a role manager as well.
If you are using Web SSO to connect to an external identity management system, you must also register an HTTP module for the Web SSO.