Nylon line is the most common line available, but it tends to wear out and break quickly on anything but lighter duty jobs. Manufacturers have come out with a variety of alternatives for heavier duty jobs, however. Some examples include reinforced composite nylon, internally reinforced nylon, and aluminum additives for added strength.
Although not line in the technical sense, some of the beefier string trimmers can work with nylon, or even metal, blades. All of these options improve the durability of the light gauge nylon string, but if the vast majority of your trimming work centers on light grass clipping, you can probably get by with cheap, simple nylon line.