What is a zigzag stitch?

A chainstitch uses one thread (needle thread) which is looped around itself after the needle pushes it through the fabric, forming a stitch that looks like a chain on the underside of the fabric. A lock stitch uses two threads. The needle thread is above the fabric and the bobbin thread is below the fabric. After the needle pushes the upper thread through the fabric, the hook loops the bobbin thread over the needle thread, "locking" it so it will not be drawn back through the fabric when the needle is raised. Zig zag refers to the stitch where the needle alternates left and right as it sews. A zig zag stitch could be either a chainstitch or a lockstitch.