Alpha - Full development stage. Most of phased developments added. Still bugs to be dealt with.
Beta - Phased development completed. Most bugs dealt with. Testing cycle
gamma - release cycle. Testing and minor bug fixing. All major bugs dealt with.
Stable/Release. Development/Testing cut off. All new development on next release candidate.
A lot of development now have RC - release cycles to replace gamma releases. e.g php now at 5.2.5rc1. Also only major releases go through alpha/beta. Sub-releases generally go through beta/gamma/rc
Google were in 'beta' stage for a long time, which in reality was post-gamma, but was still having lots of production ready things added to it so they used beta because it sounded like they were doing something.
Funny really, I was shocked at my last job when the development manager didn't know there was a gamma stage!
S