The spelling difference is small but the pronunciation difference is not — "advice" ends with a hiss, "advise" with a buzz. Say them aloud and they are plainly distinct. The written error persists mainly in quick typing, and it is common enough in professional email that it has become a minor tell.
A noun. Guidance or a recommendation — the thing itself. Never takes a plural "s."
A verb. To give guidance or a recommendation.
Quick way to remember
Say it aloud. "Advice" rhymes with "ice" and is a thing. "Advise" rhymes with "wise" and is an action. If you can put "some" or "the" in front of it, you want the noun.