Martin Evening explains the Lightroom catalog
Photo Issues
Adobe Photoshop Lightroom, like other database-powered image managers requires you to first import images into a catalog before you can modify them in any way. This requirement is a continual source of confusion for new users. In this article, photographer and Lightroom expert Martin Evening explains the core functionality of Lightroom's catalogue and demonstrates its advantages over more conventional image browsing software.
Source: News: Digital Photography Review (dpreview.com)