Darts are straight stitch lines that connect areas of fabric that must fit properly to the body. For example, darts may take in bulk in the center back of a man's fitted shirt. They may be placed at back shoulders to allow the shirt to be looser at the shoulder blades, but taper in up at the shoulder seam. They can also be placed on the sides or under the bustline. The top of a skirt often has darts to allow the skirt to be full at the hips, but taper in at the waist.
A dart starts and stops without creating any puckers in the fabric. In other words, the dart lies flat on the fabric. When sewing a dart, careful tapering from the largest part of the dart, to the end of the dart, is important. If you abruptly end a dart, you will get puckering. A dart is made by folding the fabric, and then sewing from one end of the dart to the other end of the dart.