It's a stage in software development.
Alpha stage is when the software is in the beginning stages of development and design. At this point it is buggy, not every feature is in place, and things may not work at all.
Beta is the stage where the software is stable in the development house and is time to allow the general public to take a whack at it and to report the bugs that the developers may not have found because since they developed the software, they have a certain way of doing something. Allowing the public to take a crack at it, allows the developers to see how the software holds up in real life applications.
Release Candidate is the stage where all the results from the Beta is in and anything wrong is corrected. This is the stage where they are reasonable sure that the product is good to go.
Current Release is the version that is now the current incarnation of the software and is available to the public at large.
In the cases you give above, Beta is AOL, Google or Yahoo letting selected members of the population take a crack at the latest version of the software under development.