Incrementally save registration custom fields

During user registration how can I save user data entered multiple times during a long registration process? I have 5 tabs of custom registration information (actually an application process) and would like the system to save the data entered after each tab has been completed. "When the user selects the advance to next page button".

Does the system currently save after each field has been entered, each tab completed or only when the registration has been completed. If the registration data is only stored in the database when the registration is complete - what other options could be created to save the data after each tab is completed?

Todd