A serger, also known as an
overlocker, is a special industrial-type of
sewing machine that sews with up to 4 threads at a time and also uses a small
blade to cut the edge of a
seam as it sews. Two threads (called the
loopers) loop over the edge of the fabric that the blade has cut, and two
needles sew them in place with the other two threads. Thus, a serger finishes a seam all in one step. It is a huge time-saver for garment construction, and almost all "professional" garments are constructed using one. Just turn a
store-bought t-shirt inside out and look at the seams, and those are what standard serger seams look like.