There are many layers to garments, the layers underneath the main top fabric that we see are:
Although not used on every item of clothing that we may wear, these are very useful for constructing some garments and craft projects, here is some more information on these fabric layers.
Linings are usually separate layers of silky type fabrics that are sewn into a garment, such as a coat, to hide internal seams and dart, and also to make the garment feel more comfortable to the wearer. When using linings it is best to ensure the fabric you use is as washable as the outer fabric. I have washed 'washable' coats before to find out that the lining shrinks where the outside didn't pulling the coat out of shape.
Underlinings are used in many ways. They can hide internal work from a flimsy sheer outer fabric or add strength and weight to light fabrics. Lacy tops will usually have a soft satin type underlining. Sometimes the underlining is attached to the top fabric before the garment is constructed but can also be stitched in place after all work is complete.
Interfacing provides stiffness and support to fabrics. This is very useful in making things like bags and stiffening shirt collars. Button h*** areas sometimes need extra strength and knitted garments can stretch out of shape so extra fabric support can be useful here also. They come in light, medium and heavy weights to suit the weight of the main fabric that needs to be supported.
There are two types of this support fabric:
Interlining is a seperate layer of fabric that is inserted between the outer fabric and any linings. It can be used to add warmth and padding or to strengthen a fabric, and usually only on the main trunk of a garment rather than the sleeves.