Fly fishing all started with floating lines only, but those days are long gone. You now also have sinking lines, and within that category there are different types.
A sinking lines has an indication that will explain how fast it sinks. These are the most common indications:
Slow intermediate — sinks about 1 inch per second
Intermediate — sinks 1-2 inch per second
DI3 — Sinks 3 inches per second
DI5 — Sinks 5 inches per second
DI7 — Sinks 7 inches per second
DI9 — Sinks 9 inches per second
Because of the indication you can start counting when the line hits the water. By multiplying the amount of seconds by how fast the line sinks you'll know more or less how deep it is. This is useful for instance when figuring out how deep the water is when you want to fish close to the bottom. Another advantage of fishing like this is that you can 'scan' multiple layers of the water. Start shallow, and if that doesn't yield results you increase your depth by letting your line sink longer, and then repeat. It's a great way of figuring out where the fish are when you don't have any visual indications on the surface.