How are snails different from slugs?

Snails and slugs are both members of the class Gastropoda, but they differ primarily in the presence and development of their shells.1

• Snails: These gastropods possess a distinct shell into which the animal can generally withdraw for protection against predators and desiccation. The shell is typically coiled in a spiral pattern and is secreted by the mantle.123
• Slugs: These are essentially snails whose shells have been reduced to an internal plate or granules, or have been lost entirely through the course of evolution. This reduction often occurs in humid environments where the protective benefits of a shell against drying out are less necessary and where calcium—required for shell construction—may be scarce.4153

While both groups share many biological characteristics, such as the use of mucus for movement and protection, the absence of a substantial external shell is the defining feature that distinguishes a slug from a snail.46