A museum is distinguished by a collection of often unique objects that forms the core of its activities for exhibitions, education, research, etc. This differentiates it from an archive or library, where the contents may be more paper-based, replaceable and less exhibition oriented. A museum normally has a collecting policy for new acquisitions, so only objects in certain categories and of a certain quality are accepted into the collection. The process by which an object is formally included in the collection is called accessioning and each object is given a unique accession number.
Museum collections, and archives in general, are normally catalogued in a collection catalog, traditionally in a card index, but nowadays in a computerized database. Transferring collection catalogues onto computer-based media is a major undertaking for most museums. All new acquisitions are normally catalogued on a computer in modern museums, but there is typically a backlog of old catalogue entries to be computerized as time and funding allows.