On a schooner, the forward most mast is shorter than the aftermost mast. The tallest mast is always the main mast. The forward mast is called the foremast, and the other mast, the aftermost mast, is called the main mast. If you are square rigged, the lowest, largest sail is called a course. In order to tell someone what sail to do what with, you have to locate the sail. Sails are named according to the mast they are on. The forward course, the one on the foremast is the forecourse. The aftermost course is on the main mast; thus, it is the main course.
Above those, you have the fore topsail, and the main topsail. On a square rigger, you would then have the upper fore topsail and the upper main topsail. Following that, you have the fore sky sail and main sky sail. And, then, you would have the fore and main moon sails. If you are in very light air or heavily laden, there are temporary, portable sails that are hung on their own small yards off the fore and main yards to give you extra
sail area. These are called dunnage sails. Most schooners are fore and aft rigged. Thus you have the foresail and the main sail. Sails forward of the foresail are refered to as jibs normally. Jeff has well described the
triangular sails above gaffed fore and main sails. They are topsails. Again, they are named after the mast they are on. That is, the fore topsail, and the main topsail. In a puff, the vessel can be depowered by simply allowing these topsails to drop behind their respective, lower, larger sails rather than securing them. This is called "scandalizing".