Difference between java servers

Hi portal gurus,

I want to know the difference between the portal server ,web AS java in R/3,

Will the portal server will able to call the RFC in R/3 server which is having only the ABAP stack.

when the "Web As java" in R/3 comes in to picture when we are using the Enterprise portal.

I think this is basic question , but i want to get the clear picture on this scenario

Thanks

prasad