The ancestors of these organisms were the most primitive members of the clade. In other words, organisms that we would put in this division were probably the ancestors of all the other plants. There are some 7000 species living today. Although some of the multicellular forms are large, they never develop more than a few differentiated types of cells and their fertilized eggs do not develop into an embryo.
Green algae are an important source of food for many aquatic animals.
These are fairly simple plants that do produce a number of differentiated cell types and whose fertilized egg develops into a distinct embryo. Some 16,000 living species are known. Most grow in moist places. Link to an illustrated discussion of mosses. The members of this group are often called club mosses .