The word slug basically means slow. Supposedly it takes a slug or snail 115 days to travel a mile. I find this difficult to believe, since they sure can rapidly move through a greenhouse, leaving havoc and destruction in their wake. Slugs can tear pansy flowers ragged and demolish the foliage of a beautiful hosta, ruining its appearance for the rest of the season. Slugs and snails eat bulbs, roots and the most toxic plants known to humans, including poison oak and poisonous mushrooms.
Slugs even eat each other, in addition to animal droppings and other foul, smelly things.
Although slugs and snails are terrible garden pests, they are not insects or bugs. They are mollusks, kin to oysters, clams and other shellfish. Snails differ from slugs in that they have a shell in which to hide. A clever device to have, since snails can, when faced with unfavorable circumstances, go dormant in their shell for up to four years. A slug is a snail without a shell.