[Update: This article was written for Aperture 1.1, so does not mention the more advanced importing features that 1.5 offers]

The first time you meet Brown Folders in the Aperture library is when you import existing images. Brown Folders organize content inside Projects, so they are always under them in the hierarchy. Contrast this to Blue Folders that organize Projects and other content above Projects.

For this example, I'll work with a single folder that contains images and folders. The folder itself looks like this:

Inside it are three folders and three images. If I open it I get this:

And if I open all of the subfolders I get this:

There are several ways to import this content into the library and the results differ, sometimes quite radically. In all cases, however, you can either import to a new Project by selecting Library, or into an existing Project by selecting that Project.

First lets do an import using the import button (down arrow in the top left of the Aperture window). That opens the Import pane. By selecting the drive you can navigate to the Photos folder. You see just the top level three images. Click Import All. Here is the result:

Only the top level images were imported. You can now rename Untitled Project and repeat to import more images from lower-level folders, one at a time. This will create individual Projects each with just images inside. Or you can leave the project selected and keep adding the folder content to it. This makes sense if you want to flatten your image structure because you have everything well keyworded. Doing this a few times results in this:

Still one Project, but more photos now. You would think that to divide those up into some sort of hierarchy Brown Folders could be used, but you would be wrong. Right click on the Project and add a new folder:

Drag some images from the Photos Project in and --- it does not work. That is because you cannot have photos in the library not inside some sort of container: an Album, Light Table, or Gallery. Lets try again, and this time put an Album in there and drag some images in:

This time it works. All the images are still in the Project, the Album just has a subset of them. You can add each image to as many Albums as you like. And once you have a few Albums, you will want to organize those using Brown Folders:

That's what Brown Folders are used for. Notice that Brown Folders don't display their content like Blue Folders do: clicking on one will show nothing except an icon that says Folder.