Alot of people have a hard time understanding where their images are in the Lightroom environment, what the catalog is, and how it all fits together. This is completely understandable, as the terminology is confusing.
Think of a public library — you have stacks and stacks of books everywhere — the main floor, 2nd floor, 3rd, …., in the basement, and even offsite. And you have the centrally-located card catalog (or web catalog), which helps you find and make sense of those books. The catalog has an entry for each book, with its location in the stacks, what the book is about, copyright and publication information, a picture of the front cover, etc, etc. When you want to work with books, you don’t go to the stacks and walk along them until you find what you want — you go to the catalog. Furthermore, if you put a paperback that you bought at the grocery store in the stacks, it is as if it is not there — it is not accessible to people until the librarian creates an entry in the catalog for it.
So here’s the analogy: your images are like the stacks of books — they sit on your hard drives in folders however you decide to organize them. The Lightroom catalog is just like the card catalog — it is a centrally located collection of information about each of your images that you have taken the time to create an entry for. By default this catalog is located in a “Lightroom Catalog” folder in your Pictures folder on your internal hard drive.
Contrary to what you would think “import” means, when you “import images” , you are not moving your images — you are simply creating an entry for each image in the centrally-located Lightroom catalog. Once your images have been entered into the catalog (i.e. imported), you can see them in Lightroom, and you can work on them. You may have thousands of images on your hard drive(s) that you don’t see in Lightroom — just like the paperback that was put in the stacks, they aren’t accessible to you through Lightroom until you enter/import them into the catalog.
But, you say, the Import dialog asks you if you want to copy or move your images, so importing must move your images, right? No — the fact that the Import dialog allows you to move or copy images (usually used to copy from a memory card to the permanent location of your choice on your hard drive) is simply a convenience to you — it is not the import itself, which is simply the creation of catalog entries.
So what information is stored in an image’s catalog entry? The location of the image, metadata (data about the image – more about this in another post), keywords, ratings and labels you have assigned in LR or Bridge, jpeg previews of the image, and what I call the Develop recipe: a set of instructions representing the enhancements you make to your image in LR.
A Lightroom Catalog Entry for an Image (It doesn't really look this way, but carrying the analogy along, think of it as a card in the card catalog)
Note that when you look at an image in LR, you are looking at a jpeg copy/preview of the image that LR has added to the central catalog — just like when you see a picture of the book cover in the public library web catalog, you are not looking at the actual book. This way LR (the librarian!) doesn’t have to go get the image from the hard drive every time you want to look at it.
So let’s carry the analogy further: in the public library, it really does not matter how the books are organized in the stacks — alphabetical by author first name, the dewey decimal system backwards, etc.. — as long as the card catalog can communicate to you where a book is located. Same with LR — LR does not care how you organize your images on your hard drive, and it doesn’t help you to organize them — it is up to you to decide on a folder structure that works for you. (For example, a Pictures folder with year folders within that, and shoot folders within the year folders.) Lightroom would be just as happy if you dumped all your images on your desktop, as it will simply record that that is where it should go to find them — but we know that this would be a mess for us in other ways.
One of the things you may have heard about Lightroom or observed yourself is that it can do searches very, VERY fast. I can do a search of all 20,000 images in my catalog for just winter tree images shot with my wide angle lens, and before I snap my fingers, LR will have them displayed. Why is this? Because LR searches the central catalog, rather than going out and looking for the images on your hard drive(s) — just like in the public library you can find a book much faster by going to the card catalog than walking up and down the stacks searching for it.
If you store your images on one or more external hard drives, you may have noticed that even when those drives are not plugged in, your images are still visible in LR, and you can do Library module work with them (assign keywords, rate, label, add metadata, sort, put in collections, etc..), as well as build slide shows and other output. How is this possible? The same way that if you can do library research at night on the web, even though the library is closed — you are accessing information in the catalog, not in the stacks.
I’m sure I will have more to say about the catalog in the future, but hopefully this is a start to clarifying what the catalog is about. If I have made any sense, next time someone asks you where your images are, you won’t answer “they are in Lightroom”, you will say where on your hard drive you put them.