Main Difference
The main difference between Snail and Slug is that the Snail is a mollusc and Slug is a common name for an apparently shell-less terrestrial gastropod mollusc.
Snail
Snail is a common name loosely applied to shelled gastropods. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name snail is also used for most of the members of the molluscan class Gastropoda that have a coiled shell that is large enough for the animal to retract completely into. When the word “snail” is used in this most general sense, it includes not just land snails but also numerous species of sea snails and freshwater snails. Gastropods that naturally lack a shell, or have only an internal shell, are mostly called slugs, and land snails that have only a very small shell (that they cannot retract into) are often called semi-slugs.
Snails have considerable human relevance, including as food items, as pests, as vectors of disease, and their shells are used as decorative objects and are incorporated into jewelry. The snail has also had some cultural significance, and has been used as a metaphor.
Slug
Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word slug is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a small internal shell, particularly sea slugs and semislugs (this is in contrast to the common name snail, which applies to gastropods that have a coiled shell large enough that the animal can fully retract its soft parts into the shell).
Various taxonomic families of land slugs form part of several quite different evolutionary lineages, which also include snails. Thus, the various families of slugs are not closely related, despite a superficial similarity in the overall body form. The shell-less condition has arisen many times independently during the evolutionary past, and thus the category “slug” is a polyphyletic one.
Snail (noun)
Any of very many animals (either hermaphroditic or nonhermaphroditic), of the class Gastropoda, having a coiled shell.
Snail (noun)
A slow person; a sluggard.
Snail (noun)
A spiral cam, or a flat piece of metal of spirally curved outline, used for giving motion to, or changing the position of, another part, as the hammer tail of a striking clock.
Snail (noun)
A tortoise or testudo; a movable roof or shed to protect besiegers.
Snail (noun)
The pod of the snail clover.
Snail (verb)
To move or travel very slowly
Slug (noun)
Any of many terrestrial pulmonate gastropod mollusks, having no (or only a rudimentary) shell.
Slug (noun)
A slow, lazy person; a sluggard.
Slug (noun)
A bullet (projectile).
Slug (noun)
A counterfeit coin, especially one used to steal from vending machines.
Slug (noun)
A shot of a drink, usually alcoholic.
Slug (noun)
A title, name or header, a catchline, a short phrase or title to indicate the content of a newspaper or magazine story for editing use.
Slug (noun)
the Imperial (English) unit of mass that accelerates by 1 foot per second squared (1 ft/s²) when a force of one pound-force (lbf) is exerted on it.
Slug (noun)
A discrete mass of a material that moves as a unit, usually through another material.
Slug (noun)
A motile pseudoplasmodium formed by amoebae working together.
Slug (noun)
A black screen.
Slug (noun)
A piece of type metal imprinted by a linotype machine; also a black mark placed in the margin to indicate an error.
Slug (noun)
A stranger picked up as a passenger to enable legal use of high occupancy vehicle lanes.
Slug (noun)
A hitchhiking commuter.
Slug (noun)
The last part of a clean URL, the displayed resource name, similar to a filename.
Slug (noun)
A hindrance, an obstruction.
Slug (noun)
A ship that sails slowly.
Slug (noun)
A blow, usually with the fist.
Slug (verb)
To drink quickly; to gulp; to down.
Slug (verb)
To hit very hard, usually with the fist.
“He insulted my mother, so I slugged him.”
“The fighter slugged his opponent into unconsciousness.”
Slug (verb)
To take part in ride-share commuting and hitchhiking.
Slug (verb)
To become reduced in diameter, or changed in shape, by passing from a larger to a smaller part of the bore of the barrel.
Slug (verb)
To move slowly or sluggishly; to lie idle.
Slug (verb)
To load with a slug or slugs.
“to slug a gun”
Slug (verb)
To make sluggish.