Patchwork is very often done to use up scraps of leftover fabric. It is not always the case anymore though, as people make beautiful patchwork quilts with elaborate patterns of fabric chosen especially for the quilt.
So, the patchwork quilt is made by quilting three layers together, the patchwork, the batting and the back of the quilt. You can also let the stitching follow the pattern of the patchwork or it can be something completely different.
Because of the patchwork that has to be done very accurately, a patchwork quilt is probably not the best place to start with quilting. Once you are more adapt with the other quilting techniques it will be easier for you to create a beautiful patchwork quilt.
It may also be a good idea to do a few smaller patchwork projects first to master the art of patchwork. Why not start with a patchwork pillow and see how it goes. If you get that right it will be easier to move on to a patchwork quilt.
To do a patchwork quilt is a work of art. So, in answer to your question: "What Is A Patchwork Quilt?" I would say it is a beautiful piece of art that can also have a practical use.