One of the things Lightroom does in the background is make previews of your photos. You can choose what size it will make them, if you zoom in on an image it will generate a 1:1 preview on the fly, and you can also tell the program to pre-render 1:1 previews of an entire shoot.
You can manually kill all the 1:1 previews, or have them automatically deleted on a given schedule as I have here.
But what about the scores of smaller preview files? Its not hard to very quickly accumulate a very large number of these. A look at the properties of my Lightroom Previews folder reveals very nearly 35,000 files!
The Previews folder is easy to find. It should be stored alongside the matching Catalog file. Also, if you have multiple Catalogs, you’re going to have multiple Previews folders too.
On the huge hard drives we enjoy a few gigs of previews isn’t a big deal, but if you’re running on a notebook or netbook with limited drive space, that could be some precious capacity tied up there.
Ensure that Lightroom is set to delete the 1:1 previews on its own, and if you need to recover space right now you can also choose Library->Previews->Discard 1:1 Previews from the menu to manually kill them
And in my experience it is safe to delete some or all of the files and folders in the Previews folder, just ensure that Lightroom itself isn’t running.
One final point is that backing up all the preview files is unnecessary, and will make a backup run much slower as so many files have to be scanned for changes.
In most backup programs, including AllwaySync which I use, its possible to define a folder, file name or file type to be ignored during a backup. I only discovered this recently and the speed of my backups has improved dramatically.