String trimmer heads are metal, plastic, or nylon shears with a cutting head that oscillates from one clip position to another. They can be mounted at the end of a string trimmer shaft and may cut lawns as well as take care of weeds, brush, and vines.   Other names for trimming heads include weed whacker attachments and line trimmers. Many people call them weed-eaters which has become a generic term for any small handheld powered cutting tool used around grasses or other ground covers.