Hornworts are a type of bryophytes. They are non-vascular plants which do not have tissue that moves water. The name "hornwort" come from the horn-like structure that grows on these plants. This structure is the sporophyte. The flattened, green part of a hornwort is the gametophyte.
Like all bryophytes, hornworts have alternation of generations. In this cycle, the plant has two forms: the haploid gametophyte and diploid sporophyte. The gametophyte is the green plant body that you most often see. The sporophyte is the "horn" that sheds spores.
Hornworts grow all over the world. They grow well where it is damp or humid. Some species grow in large numbers as tiny weeds in the soil of gardens and cultivated fields. Some tropical species may be found growing on tree bark.