Snails and slugs are all gastropods. The advantage to being a slug rather than a snail is that if there is an environment that has plenty of moisture but very little limestone (think the Pacific Northwest) slugs don’t need a shell to keep from drying out, but snails can’t find enough calcium to build a proper shell. Thus being sluggish can be a good thing at times.
Then, since they can’t pull back into a shell for protection, slugs have the ability to squeeze into the tiniest cracks to hide, and they put out nasty mucus that can actually kill some predators and certainly makes then taste very bad.
And actually, some slugs do have shells. They are just very small and covered over by a part of the body called the mantle.
Slugs also have behavior – they fight over the best hiding places by “licking” each other with a toothy structure that looks like a tongue but feels like sandpaper. They also seem to remember bad happenings for about seven days and so avoid the situation or place that caused them for that long.
I am a systematic terrestrial malacologist who used to study slugs so that’s why I both know and care about this subject. That plus $5.00 will get me a cup of coffee at Starbucks.
BTW, you can eat slugs if you wash them well first, to get the mucus off. A friend of mine used to make banana slug bread when he was in grad school.