Everyone is going to need and use a typical Weight Forward Floating Fly Line, also referred to as a WF line most of the time. These lines will perform a variety of operations from dry flies, to nymphs, to streamers. You can use them for trout, bass, steelhead, carp, panfish, or just about any other species. You can use them in saltwater as well as freshwater, but if you are in a tropical environment you will probably want a fly line specific for the warm water.
These lines are generally Weight Forward which is most popular. This means that the fly line has a bit more weight in the head making it easier to cast at reasonable fishing distances. These are most common. Fly rods that are more flexible often perform best with a Double Taper fly line. These are less common but favored by anglers that want to present dry flies delicately on flexible rods.