Snail, Snail, put out your horn,
I'll give you bread and barleycorns.
-- Traditional child's rhyme.
Generally speaking, a snail is a mollusk, class gastropodia*, of any of a number of families. They have a soft body covered by a shell. They are usually nocturnal. That's really all you can say on snails in general; 'snail' is not a very specific term. Annoyingly, this doesn't prevent people, even biologists, from using it as if it were a well-defined term.
Snails live all over the world, from the arctic to tropics. They come in all sorts of varieties. Many are edible, some are poisonous. Some bite, some give birth to live young. Amongst themselves, they contain all sorts of weirdness. They are often classified by the environment by which they live, salt-water, fresh-water, or on land. This is not a biological classification, but one of convenience.
Land snails: These snails have some specific adaptations so that they can live in a (comparatively) dry environment. They produce a layer of mucus to coat their bodies, helping keep the water in. This slime also helps them climb and hang on steep, even vertical, surfaces. Most land snails have lungs, but some manage with gills. Land snails have better eyesight in comparison to smell than do water-dwelling snails; connected to this, terrestrial snails have eye stalks.
- The infamous French dish of escargot is a terrestrial snail of the family Helix.
- Giant African Snails are often kept as pets (snails of the family Achatina). They can get over a foot long and weigh up to two pounds.
Fresh water snails: It's interesting to note that many fresh water snails actually have lungs, and must come to the surface to breath (for example, members of the families Lymnaeidae and Physidae). Many fresh water snails are actually descended from snails that had adapted to life on the land, and then moved back to the water. Some of these have re-developed gills, while others have not.
Us humans have a great interest in fresh water snails, because various species can carry the schistosomiasis parasite, a major health concern in areas of the world where indoor plumbing has not yet become common. We also like to use some species of fresh water snails for our aquariums (Apple snails, Trumpet snails and Ramshorn snails).
Marine snails: Salt-water snails are the largest group of snails, best known to the average human through the bright spiral shells that wash up on the shore. They are also harvested as food, particularly abalone and conch. Don't try gathering them yourself without reading up on the subject; some marine snails (family Conidae, genus Conus) secrete venom, which can be fatal to humans.
* Occasionally, the class of Gastropoda is said to consist of snails, indicating that, technically, slugs, limpets, and conchs are all types of snail.
And here's a little snail superstition, for the lonely noder. In the mid-1700s, young women (and perhaps men too) would set a (live) snail in the (cold) ashes of the fireplace, and watch its track. In theory, the snail's trail would resemble some letter, and that letter was the initial of one's future lover.