The simplest mechanism is a file index. This is just a static list of the files included in the archive. User's can search this list and choose a set of files to retrieve. Many archives are of this form. The typical anonymous FTP archive uses the directory hierarchy to provide the file index.
Many missions provide an observation index which includes direct pointers to the observation data.
In using those elements alone, one has a level 2 archive. The typical level of archive access that astronomers now demand is what we term a level 3 archive. While a level 2 archive had a static index of its contents, a level 3 archive has a database system which allows users to make queries about the contents of the archive. Once the number of files maintained in the archive gets large and the number of different types of data multiplies, it becomes very difficult for many users to find data using a static index. In moving from level 2 to level 3, the user's interaction with the archive catalog goes from dealing directly with the index to dealing with the index through a database intermediary which interprets the user's astronomical requirements.