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 all the rest was the future.
Because Catalog's resources were so great, however, the collection reached back and forward through time so far 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 at the collapse of the universe. The bugs' continued feeding on the Catalog-generated timelines was a serious thread to the stability of the web of time.
The Doctor was able to end 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 staff of Catalog to clean up — something he estimated would take about a century. (IHP: Hunger from the Ends of Time)