Taxonomy
Sea hares belong to the gastropod class of mollusks, specifically the super family Aplysoidea. Sea snail is a much less specific term, more related to the appearance of the animal than its scientific classification. The animals known as sea snails belong to various groups of gastropods with shells, from a variety of families.
Physical Characteristics
Both sea snails and sea hares have shells. In sea snails, the shells are visible. Sea hares mostly do have shells, but these shells have become internalized. The shell of sea snails is asymmetrically coiled while that of sea hares is thin and flattened. Both shells mainly consist of calcium carbonate. The two types of mollusk share the typical gastropod structure. They have soft bodies, a muscular "foot" which is used for locomotion, relatively simple eyes and a rasping tongue for eating. Gastropods have brains and can learn simple tasks, but their brains are not as complex as those of other mollusks such as octopi, .
Diet
Sea hares are generally herbivores, grazing on sea grasses, seaweeds and unicellular algae. Many species of sea snail share the same diet, although some are carnivores.
Habitat
Both sea hares and sea snails live on the sea floor, usually among rocks, corals or vegetation. They are able to swim to a limited degree but are certainly not adapted for life in the open oceans. These animals generally live in well-lit, coastal waters. Creatures that depend upon vegetation and algae need a good light source to sustain the food supply.
Care
Marine tanks are not easy to maintain, and both sea hares and sea snails require very specific conditions. The exact needs depend on the species, but generally they require a large, established tank and often supplemental feeding of suitable algae or other vegetation. Some species of sea snail are hardy enough to be a suitable animal for beginners, but you should have experience maintaining a marine aquarium before acquiring sea hares.