Arnica is a plant that is part of the sunflower family. It typically gets big yellow flowers that attract butterflies. The flowers have a slight aromatic smell. The entire plant has a strong and distinct pine-sage odor when the leaves of mature plants are rubbed or bruised. It mainly grows in Europe and Siberia but there are some species that also grow in the temperate regions of North America. The genus Arnica has over 30 species branching off of it. Arnica montana is the species most known for its medicinal properties.
Arnica can be poisonous if taken internally. There are some homeopathic remedies that use highly diluted arnica in their oral formulations to treat such things as gastrointestinal problems and eye problems. I personally would not take any arnica orally. I think if nature makes something poisonous it is as a way of telling you not to eat it. I think the risks outweigh any potential benefits that you could get from such small doses. Canada has banned the use of arnica in oral formulations.
Arnica is much safer as a topical remedy. It has been widely used in ointments for the treatment of sprains, bruises, and sore muscles for centuries. It is a favorite of athletes. It has also been used to treat osteoarthritis. It is also used to treat pain and inflammation from post-operative surgeries.