MIDAS catalogues are best described as a list/collection of any of the data structures images, tables or fitfiles. If a catalogue is enabled (cf. Chapter 3 of the MIDAS Users Guide, Volume A) new entries of the relevant data structures are automatically added. Entries in a catalogue may then be referenced also via that entry number instead of the physical file name.
The MIDAS command language provides the functionality to exercise any single application or complete MIDAS procedures on all entries in a catalogue instead of only an individual frame. Therefore, it is usually not necessary to handle catalogues in a user application. Only in specific cases, e.g. where the input to your algorithm is a whole list of images, are the catalogue interfaces needed (an example of this is the program to read/write FITS tapes).