Here's how someone described it over a decade ago 4u60

Ecchi as a manga/anime category generally refers to something having sexual references and/or turn-on scenes, typically stuff like boob close-ups or panty shots. Risque content, basically. Smut contains sex scenes and is intended for titillation, but isn't entirely focussed on the actually sex act. Same goes for the Borderline H category. Hentai is porn, its entire purpose is to display explicit sex acts. Softcore porn could include things like smut, and contains sex scenes but no extreme fetishes or violence or anything like that.

Shounen/Shoujo Ai is basically for non-explicit stuff

A just ruler amongst tyrants