Quote Originally Posted by MMorgan View Post
Interfacing basically is just a backer that helps stiffen up the material its attached to right??? (I know NOTHING about sewing LOL learning this stuff as I go along)
Interfacing is typically used for a few reasons, either singlely, or in combination.

1) it's used to stiffen, as in dress shirt cuffs and collars.
2) keep fabric from stretching. Often used around armsyce (the hole the sleeve is attached to), neck holes, cuffs, pocket attachments, zippers, etc.
3) strengthen the fabric, where something is attached (a button, for instance)
or a hole cut in it (buttonholes, pocket slits)
4) change the way the fabric drapes or hangs. For instance, good quality men's suits have a piece of hair canvas (same stuff used in traditional kilts, more or less) in the lapels. It's shaped into a curved shape, and the bottom piece stiched to it (using a special stitch called a pad stitch) while in that curve. (There exist machines to do this; they're fantastically expensive, on the order of $50,000, and you need two) The stitched in interfacing keeps the lapels in their proper curved shape.
5) keep fabric from holding a shape other than its intended one.

Of course, these reasons are often used in combination. The front placket of a dress shirt may have fairly stiff interfacing in it, which stiffens it, but also serves as reinforcement for the button holes, keeping them from pulling out of shape. A casual shirt, made with out stiff collar, cuffs, and plackets, is likely to have interfacing in all the same places. It may be of lighter weight, made of different material -- a strechable knit, instead of stiff non-woven, for instance -- or attached differently (sew in, instead of fused). But, if the shirt's well made, it's still there.