A catalog is how Lightroom tracks the location of files and remembers information about them. It’s like a database that contains a record of your photos. This record is stored in the catalog and contains data, such as preview information, links that indicate where the photos are located on your computer, metadata that describes the photos, and editing instructions recorded in the Develop module. When you rate photos, add metadata and keyword tags, organize photos into collections, or remove photos from the catalog—even when the original photo files are offline—the settings are stored in the catalog.

All of this information enables Lightroom to give you flexibility in managing, identifying, and organizing photos and media files. For example, if you’re photographing on location, you can import photos into Lightroom on your laptop computer, and then later move the original photo files onto writable media or storage devices, and continue organizing and managing your photos without the worry of filling up your laptop’s hard drive with all your original photos. You can then transfer the catalog to your desktop computer, preserving the changes you make and keeping track of where the photos are stored. (In Lightroom 1.0, catalogs are called libraries or library databases.)

By default, Lightroom loads the most recent catalog. You can open a different catalog by choosing File > Open Catalog, or you can determine which catalog is opened in the General preferences.