Fusible web is a man made fiber that will melt when heated. When placed between two pieces of fabric, the melting action of the fusible web causes it to fuse the two fabrics together. It is not woven or knitted.
Fusible web is available in rolls, similar to tape, in various widths, by the yard in the interfacing department of most fabric stores and in pre-packaged pieces almost anywhere that sells sewing notions.
To use fusible web, the fabric should be washed first, as you would pre-shrink the fabric, because the finishes on the fabric will prevent a permanent fuse to take place since the fusible material can not melt into the fibers. If the fabric has not been pre-washed, it may seem like the fusible web is adhering but it will work it's way loose as the finishes come off the fabric.
Always read the directions that come with the fusible web you buy! The instructions will be on the package or on the paper or plastic that is rolled with the fusible web if you purchased it by the yard. Test your iron on an inconspicuous area, or on comparable scraps if you have them available to prevent ruining your garment.
Fusible web is available in various weights. Use the weight that is closest to the fabric you will be fusing. If you are fusing a light weight fabric such as a blouse weight cotton, choose lightweight fusible web.
If you are fusing a heavy such as denim or canvas you want a heavy weight fusible web. In a pinch, you can also use multiple layers of fusible web but it does not work exactly like a heavy weight fusible web so you must use caution and pay close attention that all of the layers have melted to totally fuse the fabric.
Although it may be tempting to use a heavy weight fusible on everything to make sure it stays "forever", using a heavy weight fusible on light weight fabric is not advisable because the fusible web may melt and seep through the lighter weight fabric. Heavy weight fusible is also known to gum-up the sewing machine needle. Cleaning the needle can work but do not use an abrasive to clean the needle which could leave burrs on the needle and cause other problems. Sometimes a cotton ball soaked in rubbing alcohol works to clean the needle or simply picking the "gum" off the needle without bending the needle.
Many types of fusible web come with a special paper backing. Patterns can be traced directly on the back side of the paper for applique work as described in How to Make an Applique With a Coloring Book Picture. I always save the paper after I peel it from the fabric, using it to protect the ironing board cover and iron, similar to a press cloth on top of what you are pressing but under what you are pressing.
Never iron directly on the fusible web. It will melt all over your iron! I f you do not have the backing paper, parchment paper or freezer paper can be used as a buffer to prevent fusible web from melting on your iron. Rolls of fusible web make an excellent addition to your emergency supplies for quick hem repair. How to Hem Jeans and Casual Clothing