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:
- A reference to where the photo is on your system
- Instructions for how you want to process the photo
- Metadata, such as ratings and keywords that you apply to photos to help you find or organize them
When you import photos into Lightroom, you create a link between the photo itself and the record of the photo in the catalog. Then, any work you perform on the photo — such as adding keywords or removing red eye — is stored in the photo's record in the catalog as additional metadata.
When you're ready to share the photo outside Lightroom — upload it to Facebook, print it, or create a slideshow, for example — Lightroom applies your metadata changes, which are like photo-developing instructions, to a copy of the photo so that everyone can see them.
Lightroom never changes the actual photos captured by your camera. In this way, editing in Lightroom is nondestructive. You can always return to the original, unedited photo.