Storing data across calls

Hi all,

Can anyone give me a simple description of the following?

1)Request (IPortalComponentRequest)

2)Session (IPortalComponentSession)

3)Context (IPortalComponentContext)

4)Profile (IPortalComponentProfile)

I am familiar with how to store and retrieve data but I would like to know

Where each stores the data?

The object scope/visibility?

When should I use each of these?

Any help would be appreciated,

Thanks

Denis.