Facing is used to finish raw edges.
Facing is a mirror pattern of the piece being faced. If you look at a shirt collar (see image above), you have the top most piece, called the upper collar.
If you lift the collar up, you’ll see another collar under there, which is called the under collar. The under collar is a facing.
The collar and under collar are sewn right sides together and then turned right side out to create a nice edge. Sleeve cuffs and button extensions are also faced.
The facing doesn’t have to be as big as the original pattern being faced. In a jacket center front edge, the facing may only be a few inches wide. Its only purpose is to finish the raw edge by being sewn and turned to the inside of the jacket.
Interfacing is placed between a pattern piece and the facing. Interfacing should never show on a garment from the outside or inside. It should be sandwiched between two other pieces of fabric. It’s only purpose is to stiffen the garment piece.
Collars are comprised of the outer collar piece that will be seen, a piece of interfacing and the under collar (facing) hidden underneath. That’s why they are so stiff and hold their shape so well.
Lining is an inner layer of fabric, or other material, to provide a nice inside finish and another option for concealing raw edges. Linings are almost always of a different fabric than the outer layer. They may be fur on a coat to provide additional warmth or perhaps a fine, elaborately printed silk on a vest or formal jacket.