Collections vs Catalogues in Lightroom
Recently during a Lightroom session I was asked what the difference was between a catalogue and collection in Lightroom. While they might sound similar, these are actually two very different parts of the program.
In Lightroom, a catalogue is sort of the backbone to the program. Lightroom stores all kinds of information in its catalogue. Everything that you do inside the program is stored inside its catalogue. This includes things like metadata or develop settings (edits) as well as settings, keywords, and presets. This is where a lot of Lightroom power comes from, because whenever you make a change to an image, that change it is just stored in the catalogue instead of actually changing the image. That is why Lightroom is called a nondestructive editor. However, this catalogue system can also cause some confusion. Because everything is stored inside Lightroom's catalogue edits that you make will not be visible outside of the program until you export the image. Plus, if you create a new catalogue, it will be as if you are starting from scratch because everything you do is stored in the catalogue.
It is also important to note that Lightroom does not actually store any of your images inside of itself. All of your images are stored on you computer, in whatever location you would like. All a catalogue does is reference your images, and keep notes about what to do with them. It might help for you to think of Lightroom's catalogue as a cabinet where all the information about all of your images is stored, but not the images themselves.
Collections are an entirely different story. Collections are an organization tool. They are cousins with folders, and they act very similar to folders. In Lightroom, a collection is just a folder that lives inside Lightroom only. Think of a collection as paperclip. If you had all of your images in folders in a filling cabinet, a collection would be a group of images that are clipped together, even though they may not be in the same folder. That is all a collection is: a grouping of images. The beauty of collections though is that you can have the same image in multiple collections at the same time. Plus if you move or delete an image from a collection nothing happens to that image on your hard drive.
To sum it all up, a collection is a grouping of images. They are just used to gather images that are related, and display them together. A catalogue is Lightroom's backbone. It is the filling cabinet that records and stores everything you do inside Lightroom. If you don't have Lightroom yet I suggest that you give it a try because it is my favorite program for photography. You can download a trial version here if you would like (FYI this is an affiliate link).