What is a Lightroom catalog?
In order to work with your images in Lightroom, the files have to be imported into the Library catalog. This catalog is a database that keeps track of the files you import, the Develop settings you apply, Map geodata you add, etc. When a file is imported into Lightroom, the catalog grabs the basic file information: the current file name, the current folder location, and any embedded thumbnail or metadata.
How do I manage my media files with Lightroom?
If you need to move, delete, or rename a file, do so in Lightroom. Lightroom is unable to track changes to your files outside of the application, doing any of the previous three actions will cause the file to become disconnected. At this point, the file will need to be located and reconnected.
For more information, please read the Catalog FAQ.
Where can I find videos demonstrating library management?
Here are some videos to help you manage your Library:
Julieanne Kost is the Adobe Evangelist for Lightroom. Here is a list of her blog entries that deal with catalogs.
If your files have become disconnected or missing, here is a list of resources about how to get them back:
Some tips on deleting images from Lightroom: