When working on a commercial game project, especially when you have a publisher, you often times will have a schedule to keep and milestones to meet. Milestones are ways for everyone to know if the game is on track or not, because certain things need to be accomplished by them. Not making a milestone can often be a horrible thing, because your publisher usually will only pay your team if the milestone has everything in it that was agreed upon. There's no standard milestone schedule, as every company is different, but some of the most common ones are as follows:

• First-playable: This is the first version of the game that can be played. Has the main mechanic of the game in and can demonstrate how it works.
• Alpha: When all the features of your game are in, known as being feature complete. Features can change slightly, and make revisions based on feedback and testing, but at this point, unimplemented things may be dropped to finish the title on time.
• Beta: The game is complete, with all assets and features completed and finished. At this point you are only doing bug testing and fixes for potential problems that would prevent your game from being shipped.
• Gold: This is the final version of the game, which you'll either be releasing, or sending to your publisher to create copies on disks, cartridges, or whatever your device uses.

Do note that every company is different and these milestones may mean different things to different people, so be sure to clarify before diving into development.