Hentai is absolute hardcore sex as Elite Hentai was kind enough to point out.

However Echii is NOT just random panty shoots thats just fan service.
Ecchi is when they show people naked but not in a explicitly sexual nature. For intance Onegai Twins the girls sit around in the bath all the time naked lots of random shoots of them nude. Ranma 1/2 also a lot or random shoots of nudity. These are both Ecchi because while there is nudity its basically softcore you just see some boobs here or there.

A lot of anime has sexual situations heck most of them at the very least have some panty shoots but unless they show the people naked its not Ecchi and unless they show them having sex and as stated earlier penitration its not Hentai. Of course most shows have a perverted episode a random episode doesn't make a whole series a ecchi its when its in nearly every episode.