Most plants that people tend to think of are vascular (they contain internal tubes for transporting food and water within them) such as most house plants, flowering plants, etc. Bryophytes have poorly developed tubes--they are non-vascular plants. And, instead of reproducing with seeds, Bryophytes reproduce using spores.

Because of the way Bryophytes transport food and water, and because they require water for reproduction, Bryophytes are small plants, and are most often abundant in wet places. However, many are quite drought tolerant and occur on trunks of trees, in forest canopies, and on dry rock surfaces.