Lightroom is to photo processing as Microsoft Word is to word processing. You use MS Word to read or edit a word document. It doesn’t store your document files inside Word—it stores your documents in a folder you select. You can take a document file and open it on a different computer that has the same version of MS Word installed.
Applying this Concept to LightroomHere’s how the “concept” of Word and Lightroom are similar. Simply put, Lightroom is a photo processor and image organizer program. It allows the viewing, organizing and retouching of a large number of digital images. It does not store your photos inside the program. I need to repeat that again. It does not store your photos inside the program. Your photos are stored on your hard drive in a folder you selected—either using Lightroom to import them or your operating system’s file management. Just like Word, you can use Lightroom on another computer as long as you have the files you need—a catalog and the images.
What is a Lightroom Catalog?
A Lightroom catalog is a database that stores a record for your photos that contains three key pieces of information about each photo: Metadata, such as ratings and keywords that you apply to photos to help you ﬁnd or organize them
Lightroom Catalogs have a .lrcat extension and can be stored anywhere on your hard drive or even an external hard drive.
Where should I store my Lightroom Catalog?
Single computer: create a folder on your fastest local hard drive, name it Lightroom Catalog and save the catalog to this location. This is the fastest way Lightroom can access the catalog.
Multiple computers: create a folder on an external hard drive, name it Lightroom Catalog and save the catalog to this location. This setup trades speed for portability—allowing access to any computer connected to the hard drive.
Once you wrap your head around the concept that Lightroom doesn’t store your actual images inside the program, you won’t make the mistake of deleting or misfiling your images.
Source: Understanding Lightroom Basics