Slugs and nails are gastropod molluscs and there are several common species in gardens.
They are soft bodied, gastropod molluscs that move along on a single muscular foot and secrete slime. They have two sets of retractable tentacles, one pair for smell and taste and one pair for eyes. They scrape their food up with a spiky, rasping tongue. Snails tend to hibernate in the winter and are unable to move through the soil whereas slugs can be active all year round both above and below ground in all but very hot, dry or frosty weather. Snails are able to climb higher when feeding as they retreat into their shells to prevent drying out. Both slugs and snails mostly feed by night.
Most gardeners despatch them on site and I admit to keeping the numbers down in spring when they can wreak havoc with young plants.