Publishers usually put the emphasis on the frontlist. And you often hear authors speak of their backlist. Occasionally you’ll hear a reference to “midlist authors.”
The front pages of a publisher’s printed catalog contain listings of their new books, especially the ones by proven, best-selling authors. When people speak of the marketing support offered by publishers, this is what they’re talking about. The frontlist advertises books in a big way to the people able to buy them in large quantities.
In the middle of the catalog are the books that are not new, but they’re not old, either. They may once have been bestsellers that have now dropped in sales, or they may never have topped charts but sold solidly. Some authors debut in the middle of the list. Less space is devoted to these, but they still get some promotion.
The back pages of the catalog are the older books that stay in print because they continue to sell.
Authors also refer to their own “backlist,” that is, the books they’ve released that are either literally on the publisher’s backlist, or that are figuratively “backlist” because they’ve been out awhile. Publishing rights to these works often revert to the authors, who then self-publish their own “backlist” books.