Tray feeders or platform feeders suspended or placed away from the ground attract traditional feeder birds such as cardinals, chickadees, titmice, and finches. When these same feeders are placed near the ground on short poles or short feeder legs they attract birds that prefer to eat on the ground. These ground birds include juncos, doves, and sparrows. A hopper feeder is essentially a platform feeder with the addition of walls and a roof. Generally hopper feeders are made out of wood.
Seed is usually gravity feed into the tray by hoppers.
Most hopper feeders hold a large amount of seed. The hopper design protects seed from the elements a little bit better then a standard platform feeder. They aren't really weatherproof though, and it is important to check to make sure that your birdseed remains as dry as possible. Hopper feeders protect against the elements a little better but they still tend to attract feeder pests like the squirrel. Platform feeders attract almost all species of feeder birds.