HOLLIS has two main parts. There’s the traditional Library Catalog of Harvard's books, journals, films, sound recordings, maps, and so on. Then there’s a huge additional database of articles, reviews, encyclopedia entries, images, etc. which come from various sources outside Harvard. Many of these are available online through Harvard's subscriptions.
You can search the Library Catalog separately, or search for "Catalog & Articles". These choices appear as soon as you start to type a search.