A catalog is a database that stores a record for each of your photos. This record contains three key pieces of information about each photo:
The way Lightroom Classic works is different from a file browser such as Adobe Bridge. File browsers need direct, physical access to the files they display. Files must actually be on your hard drive, or your computer must be connected to a storage media that contains the files, for Adobe Bridge to show them. Because Lightroom Classic uses a catalog to keep track of the photos, you can preview photos in Lightroom Classic whether they are physically on the same computer as the software.
The Lightroom Classic catalog workflow provides two distinct advantages for photographers:
Lightroom Classic offers flexibility in managing, organizing, and editing photos because your photos can be anywhere — on the same computer with the Lightroom Classic application, on an external hard disk, or perhaps on a network drive. Because the catalog stores a preview of each photo, you can work with your photos in Lightroom Classic and see your editing changes as you work. And all the while, Lightroom Classic doesn't touch your original photo files.
It’s wise to approach your work in Lightroom Classic with some forethought. You can move catalogs and photos, put photos in multiple catalogs, and combine or merge catalogs, but doing so can be confusing. In addition, links between your catalog and your photos may break. Follow these steps to plan your catalog setup and to minimize having to shuffle catalogs and photos around between computers and drives.