Most of us know what a hem is. That (typically) folded edge of a garment or sleeve. Hems are folded to finish the garment. Remember why seams are finished? The same reason goes for hems. To prevent the raw edges of the garment unravelling into the body of the garment.
Most hems are sewn to (noticed I did not say joined to) the body of the garment. Others are left alone because they already have a finished edge, like lace. Some, although rarely, are finished with an overcast or serged stitch. A few are bound similar to a Hong Kong finish.
You can stitch a hem by machine or by hand. For me, it depends on the fabric I am working with. Sheer fabrics and heavyweight fabrics I tend to hem by machine using a running stitch or running/blind stitch combination. When it comes to skirts, I also take into consideration where the hemline will fall. I almost exclusively machine stitch hemlines that are above the knee. The friction of the high hemlines will cause some hand work to wear down quickly. A machine stitch will be sturdier.
Most of the time I prefer a combination of machine and hand stitching. I will typically fold the raw edge 1/4", press, fold 1/4" again, press, then machine running stitch to secure. Then I will fold the hem up creating the desired depth and then blind stich by hand.
I seriously believe everyone should learn how to hand sew a hem. This skill will let you adjust the hem of garments, particularly pant hems, without spending extra money at the cleaners.
Main Image Credit: (cropped) Betsy Johnson skirt at NYFW S/S16 Frazer Harrison; 2015 Getty Images via Image.net