Your catalog is a database of information about your photos. It does not contain your photos.
If Lightroom is telling you a photo is already in the catalog, that means that the information about this photo is in the database, and there's no need to import it again. You can search for the photo by its file name, using the Lightroom Search Bar. In the Library Module, click on All Photographs, then press backslash (\) to make the search bar appear at the top (if it's not already there), then click on text, then type the file name into the text input box in the Filter Bar. Your photo should appear. Adobe Photoshop Lightroom Help | Find photos in the catalog
The catalog is a database. It keeps track of where your images are stored on your computer, as well as any adjustments you have made to those images using Lightroom. The catalog does NOT contain your images. Here is a link to a set of "getting started" videos. They were produced for Lightroom 5, but everything pretty much applies to any version of Lightroom.
Thank you JimHess. That Adobe TV link was a big help. I would have to assert, though, that the catalog is a compilation of all the photos I have imported into Lightroom and not just a database of all the images on my computer. And somehow I inadvertently imported every image on my computer as there are all kinds of graphic images as well as photographs in the catalog in the folder All Photographs. Also I don't have 37,000 photographs downloaded onto my computer, but I have 37,000 thumbnails in my catalog.
Anyway thanks for the link. I was looking for a George Jardine tutorial I had looked at before, that covered the same info, but for some reason has gone underground and I can't find it.
And since I am new to Lightroom and imported all the images without sorting them into folders or setting up metadata and copyright protocols, is there any way I can remove all the image links that are now in my catalog so I can start over?
the catalog is a compilation of all the photos I have imported into Lightroom and not just a database
Lightroom creates previews of the images you import, that are completely separate from the originals.
is there any way I can remove all the image links that are now in my catalog so I can start over?
Go to File > New Catalog. This will create a new, empty catalog.
Thanks, Per Bernsten. I have created the new catalog.
Thanks for the link, dj_paige