A disk cataloger is an application that records the directory structure, but not the contents, of a volume. The saved directory structure of a volume is called a disk catalog.
Since loading a disk catalog is much faster than loading the directory structure from the actual disk, searching and browsing a disk catalog is much faster than searching or browsing an actual disk.
More important, a disk cataloger can store disk catalogs for more than one volume. And, a volume does not need to be mounted on the desktop while you are browsing its catalog. This allows you to have a single disk catalog containing the library of all of your CD-ROM's, removable media volumes, and floppy disks.
Using a disk cataloger you can, in one motion, search all of your volumes, be they mounted on your desktop, in a box next to your machine, or at the office on the other side of town. And, because only the directory information -- not the actual volume data -- is saved, disk catalogs are quite compact.
Assume you have five CD-ROMs, each containing several thousand files. On one of these discs is a file that you would like to view. You have no idea which disc contains the file.
Without a disk cataloger, you would need to insert each CD-ROM individually and use Apple's Sherlock or Find File to search for the file. This can be quite time-consuming, considering how long it takes to mount, search, and eject a CD-ROM.
With a disk cataloging application, so long as those five CD-ROMs were scanned into a catalog file, you need only search that file. The search will be extremely fast, and you don't need to insert any of the CD-ROMs except for the one that actual contains the file.