Gilled snails have gills and can close their bodies into their shells with a plate, or door. Gilled snails usually lay their eggs in jelly-like cocoons above or under water. Pulmonate snails have lungs. They either come to the surface to breathe, or some breathe through their body surface while underwater. They cannot close their bodies into their shells.
Examples are pond snails and ramshorn snails.
They will clean up leftover fish foods. They will eat dead plant and animal materials, thus cleaning the tank. They do not harm live, mobile fish. They are interesting in their own right and part of the ecosystem. If they multiply too rapidly, they could die off.