OK! OK!
I'll go ahead and answer you question with a question.....What exactly are you looking for in an answer?
If you want to know how the products are made, you can google "shellac" and/or "polyurethane" and find the answer.
If you want to know how they are used as finishes, the answer is "They can be used basically the same way as a finish" but, it depends on what you are looking for as a finish. As stated before, Shellac is alcohol based and would not be a good finish for bar tops, medicine chests, or jewelry boxes as they could be exposed to alcohol in some way. Polyurethane is a hard durable finish and can be had in gloss, semi gloss, and satin and can be used on almost anything as far as chemical damage goes.
As a side note shellac is used to coat candy and pills and can be ingested. (I know, I know, and it's made from bug poop!?!?!?)
I generally use a #1 lb cut of shellac as a sealer on most woods that will be stained to prevent blotching and then after the staining I can put on more #2 lb cut of shellac or I can use the polly of my choice on top of that.
Shellac can be purchased in flakes and mixed with alcohol to the strength that you want. I generally buy mine for a paint/home store in a can pre-mixed (Zinser seal coat for example)
Rog
