Catalogs are lists of things. Think about a type of catalog that you see almost every day in your mailbox: the product catalog from a company like J.Crew or Eddie Bauer. Product catalogs list the products that companies have for sale.
Our online catalog is a list of the contents of our library. In other words, it tells you what the library owns. We call it an "online catalog" because it is in electronic format, and you have to use a computer to access it.
