Catalog was a library planet. According to the Seventh Doctor, it was "the greatest library in the universe". At the time he visited, the planet's staff had relatively recently converted their storage system. Instead of trying to locate their wealth of data across the planet's physical space, they archived it in time. Things beginning with the letters A-L were housed in the past, M was the present, and N-Z were housed in the future.
Because Catalog's resources were so great, however, the collection reached so far back and forward through time that it was a major temporal event. It attracted the attention of bugs that, according to the Doctor, were "forces of chaos" which usually resided only at the Big Bang and the collapse of the universe. The bugs' continued feeding on the Catalog-generated timelines was a serious threat to the stability of the web of time.
The Doctor ended the threat only by depriving the bugs of their food. He collapsed Catalog's collection back into real space. Suddenly, the entirety of the collection returned to physical books and records scattered across the surface of the planet. Typically, the Doctor left the Foreign Hazard Duty team called in to deal with the bugs to clean up — something he estimated would take about a century. (COMIC: Hunger from the Ends of Time!)