In a word, interfacing is used for stabilizing. It's an important part of many sewing projects, especially in garments. As the name interfacing (or interlining, as it's sometimes called) implies, it usually goes between layers of fashion fabric, where it provides invisible support.
You'll most often use it for one of three purposes:
Let's go through the list above.
Think of the cuffs of a button down shirt. They have some stiffness, right? That's courtesy of interfacing. Most of the time you use interfacing it's going to be for collars, cuffs, and waistbands. Even if using a naturally crisp or heavy material, you will need interfacing in structural areas so that they are less limp than the rest of your garment. It's all about relative body.
Similarly, interfacing can add structure to bags, costumes, or any other architectural detail. Say you want a silk evening clutch. Made out of only cloth, it would be like a pocket. It would sag and distort when you put things in it. Interfacing is what gives a purse the ability to hold a shape.
Interfacing is used to reinforce areas of fabric that are cut/punctured in some way, or that have weight pulling on it—any area in which fabric might be stressed. The interfacing will reinforce the fabric so that it won't tear. You'll sometimes find it in button plackets on shirts, and you will always find it behind embroidery or beading.
3. Stabilizing stretch
Even stretch garments have structure, and knit interfacing (which is mildly stretchy) will reinforce areas that fabric might need help holding a shape (such as around a neckline). You can also use woven interfacing to completely eliminate stretch, when necessary. (For example, if putting a zipper in a knit fabric, you must reinforce the seam with interfacing. Or if making a bag, which must be rigid, you can interface stretch fabric to make it suitable.)
There are several levels to navigate in selecting the proper interfacing. There are a lot of options but don't worry, choosing the right interfacing pretty common sense. The basic rule of thumb is, match your fabric.
In a nutshell, first you decide whether you want fusible or sew-in. The difference is just what it sounds like. One irons onto your fabric, the other doesn't. In most cases, which one you choose is just a matter of personal preference.
Next, do you want woven, non-woven, or knit? A 4-way knit fabric requires a 4-way knit fusible, if you want to maintain stretch. Otherwise, again, which you choose is basically a matter of preference. (Non-woven is ideal for things like appliqués, because you don't need to mind a grain line. This allows you to turn a shape any direction on the fusible fabric in order to get the best fit, when cutting.)
Warp-insert and weft-insert fusibles are somewhere in between woven and knit. They are knit fabrics that have had stabilizing fibers added in one direction, making them 2-way stretch. Warp-insert interfacing has crosswise stretch; weft-insert has lengthwise stretch. You can use these interfacings on non-stretch woven fabrics; they are a common, versatile choice.
Lastly, choose the appropriate weight. Most types of interfacing come in multiple weights. Choose according to the weight of the fabric you're using and whether you need a soft drape or a stiff drape. (Wovens tend to be stiffer; knit-based interfacings tend to be softer.) In craft or home decor purposes, choose according to your desired finished product. (For example, if making a belt, you'll want a heavy weight. If ironing an appliqué onto a cotton voile shirt, you'll want lightweight, so that it drapes with the cloth and doesn't create a hard patch.)
There are a few specialty materials, such as buckram and tear-away stabilizer. Buckram is essentially an open weave canvas that has been treated with stiffener. This allows it to have a rigid body without being thick or heavy. Buckram is used in millinery but is also suitable for bags, etc. Tear-away interfacing is essentially a paper that is used to stabilize embroidery; you just tear away the excess when finished, saving you from a lot of fussy trimming.
This post covers common types of materials produced for interfacing, but in sewing, the task of interfacing can be accomplished with many materials. There are goat/horsehair canvases (commonly used in tailoring), you can use regular old muslin, and sometimes even using an extra layer of your self fabric is fine. If sewing something sheer, you'll probably use organza or organdy.
Here are some kinds of interfacing in action, so that you can see the effect they have on fabric.
I've used the same lightweight cotton for each demo. On the left is un-interfaced fabric. On the right is the swatch with interfacing.
You can see how the drape of the cotton is unchanged with the lightest interfacings, has just slightly more body with the medium, and ranges from stiff to outright creaseable with the heavyweights. Below is an example (on a swatch of twill) of how tearaway works:
That's it! Go forth and interface!
As a bonus, my photography partner today: