Local library catalogues list the printed and electronic stock of a library. If they are expanded to discovery systems, they can also be used to research the content of other catalogues or databases.
The University of Trier has two local catalogues: the OPAC HS Trier as a catalogue for for the libraries on the Main Campus and in Idar-Oberstein and the eBib as a catalogue for Environmental Campus Library.
Union catalogues record the stocks of several libraries of a regional group, e. g. of the Association of North Rhine-Westphalia and Rhineland-Palatinate; metacatalogues allow you to request different (union) catalogues simultaneously with one search query.
Important: Library catalogues usually do not give you the ability to search directly for articles. Instead, you have to search for the journals or collective volumes that contain these articles.