A Serger only sews an overlocking stitch, while your regular sewing machine will sew multiple types of stitches.
A Serger works by trimming, encasing, and sewing seams simultaneously, resulting in neat and durable finishes on the edge of the fabric.
a Serger uses multiple spools of thread (cones of thread) to create a chain stitch, typically a combination of two, three, four, or even five thread