What are the two types of 1921 silver dollars?

The two types are the Morgan Dollar (common, only worth silver scrap) and the Peace Dollar (worth much more than silver scrap, especially if uncirculated)

The easiest way to distinguish between the two types is to look on the reverse, the Morgan dollar has an Eagle, wings stretched surrounded by a wreath. The Peace dollar has an eagle sitting on a rock inscribed PEACE with its wings folded.