Serger / Overlocker
A machine that trims and finishes a seam's raw edge in one pass using multiple threads and loopers, producing the stretchy, enclosed edge seen on knit garments.
Serger and overlocker are the same machine — American and British English for the same thing. Where a domestic sewing machine forms a single line of interlocking stitches, a serger uses multiple spools of thread (usually three or four) and a set of loopers to trim the raw edge as it sews and wrap thread around it in the same pass, producing that distinctive enclosed, slightly stretchy edge you see on the inside of a T-shirt.
It's not a replacement for a regular sewing machine — it doesn't do buttonholes, zippers, or topstitching — but for two jobs it's genuinely hard to beat: finishing raw seam allowances so they never fray, and sewing knit fabric, where its slight built-in stretch keeps the seam from popping the way a straight stitch would under tension. A lot of ready-to-wear knit clothing is constructed almost entirely on a serger for exactly that reason.
Threading one intimidated me for a long time — four spools, multiple tension dials, a lot more than a regular machine — but most modern sergers have color-coded threading paths and it becomes routine fast. The bigger adjustment is trusting the knife: it's cutting fabric as you sew, right at the edge, so there's very little room to correct a wandering seam line after the fact.