Collagen has 2 types. 1 extracted from plant and the other from fish. Most people claim that fish is better but if it’s in the form of powder. Fishy smell are likely to be rejected by some people.

Function of Collagen and why people buying it: It helps hold on to the abundant protein in human body. Collagen can be found in bones, muscles, skin and tendons where it provide strength and structure to it.

For more info: http://www.medicalnewstoday.com/articles/262881.php

Placenta: It an extracted contains from mammals such as sheep & deers generally. It comes in soft gel pill and enteric coated capsule. 

Function of placenta and why people buying it: Placeta activates the release of HGH (Human Growth Hormone) which vital hormone to regulate to regenerate damaged cell and also repairing wounded cells. It also helps produce collagen and glycosaminoglycans that is similar to collagen function.

For more info: http://www.wisegeek.com/what-are-the-benefits-of-sheep-placenta.htm