Slugs

I'm back blipping because I got home late after a long train journey last night, and when I did I had to put my bucket out and this slug had climbed aboard. I feel like I should know this but I don't, are snails and slugs the same thing only one has a shell? Can a slug obtain a shell and then become a snail, or a snail lose it's shell and become a slug? Some sea creatures can do that, live with or without a shell. I haven't googled it yet. Anyway I hope he found his way off of the lid of the bucket before they got emptied this morning. I thought about moving him myself but I remember hearing humans touching them harms them.