Snails are not insects. They are members of the class Gastropoda, and are more closely related to squid than to the insects that live in gardens.
Snails and slugs are both in the gastropod group, and there are members that dwell in marine or fresh water as well as on land. All gastropods are soft-bodied animals with no internal bone structure. Their shells are considered a kind of exoskeleton, but unlike insects, it is not chitinous but made out of calcium carbonate. Snails and slugs, in contrast to insects, do not have legs, and move by flexing their fleshy foot, which makes up the underside of the body.