A basic technical discussion is at The Outdoor Technical Feature by Clive Tully. A ply is a smaller thread that is used in the making of a bigger thread. Taffeta is slick and slippery. It is inexpensive, and makes it easy to slide a garment on over multiple layers. The main drawback I have encountered with taffeta is that it can retain moisture and become clammy and cool under certain conditions.
Fleeces can also work as linings, depending on the purpose of the item.
3-layer, due to the fact that most of it goes direct to manufacturers. Visit the links page for a list of outdoor fabric sources with internet access. The old stand-by method is to use a seam glue, Seamgrip being the best product available. It is now possible for home sewers to use iron-on tape. For sources of tape, go to my links page and check out the fabric sources.