Interfacing is a sewing notion that is used to stiffen fabric or help a part of a garment hold its shape. A shirt's collar and cuffs are two common ways to understand what interfacing does. It is also used in a facing to help a garment edge hold its shape but not stiffen the viewable area of the garment. Interfacing comes in a variety of forms and weights.
Woven and Non-Woven Interfacing
Woven interfacing is woven the same way fabric is woven. You will see threads running horizontally and vertically. The choice is yours unless the pattern specifies a certain type of interfacing for you to use. Test the fabric and the interfacing together by laying them over your hand together to see how much stiffness the combination will create.
Fusible and Sew-In interfacing
Fusible interfacing will have a glazed look or a spotted look on one side of the interfacing. The side with the spots or glazed look is the fusible side that when used with the heat of an iron, the interfacing will permanently adhere to the fabric. Fusbles are often used in appliquing.
Boning is a sewing notion that comes in different forms. One of the easiest ways to use boning to create a shapely seam is to encase the boning in a seam allowance.