Moss is a very simple type of plant that lacks conventional roots, stems, and leaves. It refers to any species of the class Bryopsida and is part of the division Bryophyta. Bryophyta means the first green land plants to develop during the evolutionary process. Moss is thought to have evolved from very primitive vascular plants. Moss has not given rise to any other kind of plant.

Because moss lacks traditional vascular structures of true leaves, stems, and roots, its growth is limited to moist locations. Moss is usually very hardy and grows almost everywhere, except under the sea. Moss usually grows vertically. Except for the commercially viable Sphagnum peat moss, moss is generally of little use to humans. It is also of little use to animals, although it is sometimes eaten in times of famine.

Moss is sometimes used to fill in barren habitats such as dried lakes, to provide a backdrop for other plants in gardens, or simply to add color where grass refuses to grow.

Unrelated plants can share the name moss. This includes club moss, flowering moss, carageen (a type of algae sometimes used in health foods), reindeer moss, (which is actually a lichen), and Spanish moss. Spanish moss is usually regarded as a parasite, since it often grows on other plants, such as oak trees. This moss grows in long streamers and is often seen in the Southern states of America.

The Japanese have gardened with moss for centuries. Valued for its reduced need for watering, its greenness is considered to add a feeling of lushness and serenity to Japanese gardens.

Because it needs so little maintenance, moss is often used in rock gardens or with water gardens, ferns, or ponds. With concerns about drought growing in different parts of America, moss has become an increasingly desirable alternative to high-maintenance grass lawns and conventional gardens using shade plants.