Ero/non-ero is a separate axis from bishoujo/bishonen/shoujo-ai/icky, so neither can be a proper subset of each other. It's quite possible to have an ero-bishonen game. (IIRC, PP will release one or more of these.)
Hm... My own classification, informed by:
goes something like this:
First of all, there's the "relationship". Basically, this involves the gender of the pov character, and the gender of the characters he or she can pursue. For this we have:
bishoujo: The pov character is male, the pursued characters are female.
bishonen: The pov character is female, the pursued characters are male.
shoujo-ai, yuri: The pov character is female, the pursued characters are female.
shonen-ai, yaoi: The pov character is male, the pursued characters are male.
The latter two have alternate names that would seem to be used in ero-games. A game can have multiple relationship categories, especially if its name begins with "x-change".
A second category is the age rating... that is, how much of an eroge is in it, and what sort of content is in there. Following some combination of the ratings used for manga and movies in the US, obvious ratings are:
18+ Explicit sexual content or nudity, gore.
16+ Offscreen sexual content, featureless female nudity.
Hm.. I think there should probably be some other categories here. A 13+ category for violence, and perhaps some sort of sexual awareness... not sure what that means. Non-cartoon violence should probably also warrant a 13+. An all ages rating for inoffensively boring stuff.
This is a hard category to get right.
I don't even have a name for the third category, but it contains the following values:
ren'ai - A game that emphasizes romantic love, or at least complicated love. These games move slowly towards ero, if at all.
light - Light games have light consensual relationships, moving rapidly towards towards ero. Usually, an ero scene takes place immediately after the pov character confesses. Sometimes, before.
dark - Feature non-consensual relationships.
None - No relationships involved in this game. Shoujo attack, for example, although in general such games are not covered by this system.
An open question is how much ren'ai should cover non-romantic forms of love. Despite the derivation of the term, I think most people here (and the archive) would accept a game featuring non-romatic relationships... or maybe not. Well, the archive might, anyway.
A fourth category is game structure. This describes how the game is structured and played. First up, we have the simple, story-based games:
Kinetic novels - are games where there is only one path through the story.
Limited visual novels - are games where there are multiple paths through the story, but those paths do not affect the outcome of the game.
Visual novels - are games which are DAG structured. Basically, this means that for any two events in the story, it's possible for an author to tell which will occur first, the vast majority of the time, because he or she coded the ordering explicity into the program.
Dating simulations - are games where the ordering is not pre-determined, but instead is determined by the game itself at runtime, based on some set of conditions that can only be known when the game is played. If for many pairs of events, a can come before b and b can come before a, you may have a dating simulation. Apart from the DSE demo, I can't think of a WAS DS.
It's possible to consider a game which is completely generated by the computer at runtime, but at least for now that would seem to be idle speculation.
Finally, there is "gameplay". This determines how the decisions are made. Some of these names are just placeholders, as in general I tend to sort of merge this category and the previous one in my mind.
clickity, clickity games are games where the only gameplay is dismissing text and making choices from my menus. Since I suck as a gamer, this is all I'm good at. Hence, Ren'Py.
RPG games involve some sort of stats-based combat.
It's possible to put any mechanic here... for example the Japanese Galaxy Angel games have flight-sim gameplay, IIUC. I believe Gatekeepers was a SRPG. And so on.
I'm actually not sure how to classify Summer Schoolgirls' map-based gameplay, especially because it's usually not possible to backtrack on the map once an encounter has happened.
Often, one will omit the game structure on a game with a significant gameplay aspect. So one would probably call Heikou an RPG, even though it's either an RPG-VN or RPG-DS. (I believe the former, but I'm not totally sure.)
Finally, let me qualify that I believe this reflects a systemized version of the usage of the terms in this community, rather than one the terms mean in Japan. In fact, some of the terms may be trademarks in Japan, and generic here, and others may be expanded, contracted, or mutilated beyond recognition. If they can do it, so can we.
Another Old-Fashioned Bishoujo Gamer
Supporting creators since 2004; Code > Drama
(When was the last time you backed up your game?)
"It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face in marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming" - Theodore Roosevelt