The overcast stitch is a stitch you have seen a lot. It is the less secure cousin of the overlock stitch that is creating on overlock or serge machines. It is used for different purposes and can be done by hand or by sewing machine.
The overcast stitch is a series of slanted stitches that loop or whip up and over an edge. It is often referred to as a whip stitch.
This stitch can be used to finish the edges of raw seam allowances or just a raw edge of fabric to prevent it from fraying. It is also used in repairing tears in fabric. The closer the stitches are together the more secure they are.
The overcast stitch is also used to secure the edges of embroidery. When used in embroidery the overcast is tight. It often creates a slight raised edge to embroidery. You can also overcast the edge of applique instead of using a zig zag stitch.
There are plenty of videos and blog tutorials for doing the overcast stitch both by hand and sewing machine.
Main Image Credit: Grace Christie [Public domain] via Wikimedia Commons