Floaters are plants that float on the water and are most often used for shading your pond. The shade helps to keep the sun off the surface of the pond which decreases light getting to the algae, resulting in less algae growth.
Floaters also have roots which hang down in the water pulling nutrients from the water, cleaning it in the process. Additionally, the roots act as habitat for small fish, tadpoles and other pond larvae.
These plants are generally tropical and there are purchased as an annual and thrown out at the end of the year.
Water Hyacinths and Water Lettuce are in this group and are tropical plants.