While a sloop always has two sails, a cat-rigged boat generally has only one. Note that the mast is positioned very far forward, almost at the bow, making room for a very long-footed mainsail.
The mainsail of a cat rig may have a traditional boom or, as in this boat, a loose-footed mainsail attached at the aft corner to what is called a wishbone boom.
A primary advantage of a cat rig is the ease of sail handling, such as not having to deal with jibsheets when tacking. Generally a cat rig is not considered as powerful as a Bermuda rig, however, and is more rarely used in modern boats.