Aquatic snails are important for the breakdown of dead animals and plants in water (their food source). They have a radula, which is a strip of chitonous teeth, which they use for rasping and grinding.
The secrete their own shell, growing it for their needs. They usually attach to plants or rocks in the water to keep their location, or else they could be washed away. They breathe through different methods, depending on type of snail - some have gills in their shell, some have air pockets in their shell, and some have an apparatus that is thought to be associated with breathing.
Some snails are hermaphrodites, but many are distinctly male or female. Snails have sex by an exchange of sperm packets and eggs in a gelatinous mass attached to a substrate. Raunchy, hey?