The Mendeley Web Catalog is a collection of all the references added to personal libraries by Mendeley users. As more people add their versions of the same reference to their personal library, Mendeley compares and consolidates the different versions to ensure the Mendeley Catalog version is accurate.
You can search the Catalog from within Mendeley Desktop, by clicking on Literature Search in the left pane above the My Library section.
From your search results, clicking on any result will display the details in the right pane. A banner at the top will indicate whether or not the article is in your Library, and gives you the option to save the reference.