This is the easiest line for beginners to cast and is the most common line included in quality combination kits. These lines generally produce more delicate casts to selective fish, but they won't cast as far as weight forward lines and they don't handle the wind as well. Level lines are just what they sound like, lines with no taper. The only redeeming quality these lines have is their low cost. They cast very poorly and will frustrate those who use them.
Some lines float while others sink and others have a floating body and a sinking tip.
They each have their uses. Line manufacturers put all these weights and other designations together on the line box. One line factor you won't see on the line box is a quality designation. A good fly line is smooth to the touch and has a slick feel to its surface. Good lines cast better, float better and won't wear out the guides on your fly rod like cheap ones can.