When people start collecting information they remember what they have and hopefully whereabouts it is. As more information is collected and when more than one person looks after it, that becomes more difficult so they start creating lists. As more information arrives that becomes more difficult. Professional archivists have the same problem and over the years they have developed standard ways of describing what they hold and related information about managing the material.
With the advent of computers these have been used to manage this information and specialised software has been developed for the job and for transferring information between different archives. Nowadays they have a web interface so that researchers can discover what is held in an archive prior to making a visit. This of course is the archives catalogue. Note that there is a definite distinction between the information in the catalogue and digitised copies of the information itself. The starting point is to record in the catalogue what information is held in a repository an the exact location where to find it. Digitised copies of selected information is a separate exercise and most catalogues provide a link to digitised copies.
The software we have implemented to provide a catalogue for the Lytham Heritage Group Archives is a package called ArchivesSpace which provides a lot of functionality and is free. So far we have added information about some of the information we hold taken from a very basic old computer system. Over time we plan to add informtion about more items and provide a better description.