Mosses are very small green plants.
Often moss stems branch and rebranch. Usually many mosses grow together forming a thick green carpet. Sometimes this carpet is no larger than a dime, but other times it may cover areas several feet in diameter. The mosses in the picture above were found on some soil near my place, and each plant, from root to tip-top, is not much over an inch high. There is a good chance you might spot some mosses in your own backyard, even if your backyard lies smack inside a big city.