What's the difference between themes and layouts?

I’m reviving this old discussion to ask what I see as the implicit question: using a theme to style a website is mandatory?

If I don’t want to share a theme or reuse other’s work but just use a style for a single website, then I don’t need to create a theme, right?
I can use toplevel archetypes/, data/, layouts/ and static/ directories. What about i18n/? Can I use it at toplevel as well (it’s not created by hugo new site)?