I'm very new to Salesforce and am attempting to write an application for my company which will allow our clients to login and log a case or review cases, etc, just like the "self service portal", only more customized to our needs.  From what I understand so far, in order to create a session, you must use a salesforce login.  How can I write an app that will allow my companies clients to login and interact case data, using the self service portal login, rather than a real login?

or if that doesn't make sense,

how can I recreate the self service portal from scratch?