Ok, once upon a time I had an article describing correct terms for alpha/beta/etc. So I'm going to redo it, and hopefully clean up some shit
Ok, we have a term called "prototype." Prototype basically encompasses anything pre-release in development. There is a software release life cycle. (http://en.wikipedia.org/wiki/Software_release_life_cycle) However, I've worked in this field, and it is slightly modified.
Tech Demo - This doesn't really fit into the life cycle. Tech Demos are usually for testing and displaying purposes. Some are playable, some are not.
Examples: Sonic Crackers, Robocop for PSX, etc.
Pre-Alpha - This is more committed. Pre-alpha means they have a design in mind. At this point the engine isn't fully worked out, and it only has the most basic features. If the game is built on top of another game there will be leftovers of the previous game(s).
Examples: Ecco 2 Prototypes, "Nick Arcade" Sonic 2 Prototype
Alpha - Usually the engine is finished, but the rest of the game isn't finalized. This is the point where game features will get their last changes.
Example: "Simon Wai" Sonic 2 prototype
Beta - Beta is a term used for games when the engine/features/levels/etc are all finished.
Example: Most of the chaotix prototypes dumped by drx.
RC/gold/gamma - Usually the finished product.
Example: everything in stores
Revisions: These usually go in version numbers, most of the time starting with 1.0. 1.1, 1.0a, etc are usually tiny bugfixes or tweaks.
Example: GTA San Andreas, with the hot coffee code removed