Moss balls are actually a species of spherical hair algae that form into balls when they are gently moved by waves/currents in Japanese lakes and rivers. They are a lovely addition to any coldwater or tropical tank; adding a splash of intense green and unusual texture. Larger fish love to nose them around and tiny shrimp will clamber over them searching for microscopic snacks.
They are very easy to care for - they only need low-medium light and a gentle filter system to prevent debris settling on them. Remember to turn them over every few days to allow each side access to light or you'll find the underside will brown-off in time. If they start to look more fuzzy and over-grown than you'd like, just trim them with scissors - it's like aquatic topiary!