Collection: a virtual subset of the database that is currently in use. It contains only pointers to the actual image so you can have photos exist across multiple collections. A quick collection comes handy if you need to isolate some photos to a temporary purpose, say, to print, to send to a friend, etc.
Folder: normally, a folder in Lightroom corresponds to that on the disk. Hence, you cannot have files with identical names under one folder—which makes a unique naming convention important during the import. However, the changes you make to the disk folder are NOT automatically reflected in Lightroom, unless it is a “watched folder”. In this sense, folder is not the folder as you see in, say, Picasa, but just another hierarchy view of the library.
Keyword: a property that is part of the metadata but serves as an index for searching. Since you can easily find photos taken with a certain camera, lens, ISO value etc, keyword is better reserved for subject matters: the name of the person, the location, the theme, etc. Multiple keywords can be applied to one photo.
Rating, Color Tag: in principle these are the same thing with keyword. They are part of the metadata that has only subjective value and are most often used in the preliminary selection process. What is digital darkroom? Apart from the actual developing and creative retouching, there is a considerable work load of selecting the right set of picks to work with. Anything that facilitates this task is thus useful.
Stack: a series of photo taken with the same subject, same lighting, same lens parameters, and an identical composition. This is extremely useful for professional photographers, especially portrait photographers, since they always take a stack of photos on everything. There is even an option that will automatically stack those photos taken in a given interval, say, less than 2 seconds. In my opinion, photos in the stack have a large chance to have the same white balance and tone curve setting, so it is probably desirable that this is done automatically. Yet manually do this is not a lot of work since you can paste a setting to multiple photos. The visual arrangement of stacks needs more work: its current layout not yet pleasant.
Virtual copy: (Apple Aperture’s version) a metadata set exist in relate to the actual pixel information. Unlike snapshots which exist only in the process of developing, virtual copy is the final product. This makes one thing possible: to interrupt the developing indefinitely and yet to keep your current work in progress saved.