In my opinion it's a matter of debugging

Alpha - software released that was only tested for bugs by the developer when you release compile the list of bugs (there will be alot of its complex)

Beta - well you've gotten rid of the majority of bugs

RC - It's feature request now, and smaller bug handlings.

__________________
"What everyone seems to forget is that while knowledge certainly is something - it's the implementation of knowledge that brings power" - Andres Galindo.