Line and line heads. To cut, all string trimmers use rapidly spinning monofilament line, and its flexibility offers a degree of safety unmatched by bladed trimmers. How fast the line wears depends upon its thickness and the material it's hitting. This monofilament resembles a heavy fishing line, and its diameter is one way trimmers vary. Thicker lines don't necessarily cut heavier growth, but they cut faster and are more durable.
Most heads spin a single line, but some have two lines, one from each side.
Manual heads, the most basic, require that you turn the engine off, then advance the line by hand. This type of trimmer is all but extinct. Automatic is most convenient and costly. It works by sensing the increase in rpm as the line wears and gets shorter, then releasing line until the rpm return to normal. However it's advanced, the line eventually runs out and needs replacing.