There are different types of fly lines and their ratings can be confusing. Whereas most regular freshwater and saltwater anglers pick between fluorocarbon or monofilament that is rated in pounds breaking strength, fly lines are rated by "weight," for example, 6 weight. This number does not refer to breaking strength at all. It refers to the weight of the first 30 feet of line, in grains. Grains is a traditional measurement of weight, based on what an average grain of wheat weighs.
Lines are typically either floating or sinking types with variations of both available.
There are many reasons for wanting your line to sink. Many times the fish are deep and are feeding on the bottom. In this case you need a line that will deliver some sort of wet fly that imitates what they are feeding on. This requires a sinking line or floating line with a sink tip. With new advancements in lines, full sinking line is quickly becoming less and less popular. Instead people are using WF floating line and then adding certain tips to the line that have different sink rates.