There are many worldbuilding products available to a GM, and this is one of the simplest, cheapest, and easiest to follow. It's 16 pages long, including workbook-style sections that leave plenty of room for people to write their own notes in. There's no art - this is for working with, not for oohing and aahing over.
There are five sections in World Building. The first emphasizes the logic of your world The second asks you to try to evaluate the purpose of the world you are creating as well as its place in various classifications of fictional worlds. Third, you now have a direction in which you can develop the logic of the world you're creating. Suggestions for research also come in here. Part Four is about the civilizations and mythology of your world. Finally, you are asked to focus on the workings of the world, whether fictional or game-mechanical.
The most fun part of this supplement are the worksheets that I mentioned earlier. These have several open-ended questions and a blank space for you to answer for your particular world. Keeping these documents throughout a campaign will help you improvise during game sessions and will remind you of what your game is all about when you are in the midst of a campaign. Are you still going the way you intended to go in the beginning?
Because this product is priced exactly right and is meant to be worked on rather than shelved or archived, I give it a high rating. The only way it could be better is if the worksheets had been more developed or been form-fillable, so that people who prefer to type for their archival copies could type directly on the sheet.