In The World God Only Knows, games in which you conquer girls are defined as dating sims. But in Oreimo, it is called eroge. Some forums seem to use the term visual novel a lot, even though I thought they're all similar.
What is the actual difference between each of these?
All of these are closely related and many games would fall into more than one category. However, which word you choose places the emphasis on different aspects of the game. In any case, when it isn't important to be precise, these terms are often used interchangeably.
There are a lot of other closely related terms that could be used to describe these kinds of games. I'll describe these briefly here:
A Sound Novel is a type of visual novel. The term is a trademark by Chunsoft and emphasizes the sound aspects as opposed to visual aspects. Sound novels are actually older than visual novels but because of the trademark the term "visual novel" is the one that's stuck in the long term. For older games the distinction between the two is somewhat notable, but for most modern games there's basically no difference between the meaning of the two terms.
A Kinetic Novel is a type of visual novel without any gameplay at all. That includes things like making decisions for the protagonist. Because of the lack of any player input, the story is entirely preset and the player just reads through it without any input. These are quite close to ordinary novels with added graphics, sound, and typically more focus on dialogue. Planetarian would be an example of such a game. Kinetic novels are usually shorter than other visual novels, though there are exceptions to this like Higurashi.
An Otome Game (乙女ゲーム) is a game marketed to females. It is roughly synonymous with the term Bishounen game (美少年ゲーム), which is of course directly analagous to bishoujo game except now the characters are males rather than females. A well-known example is Hakuoki.
A nakige (泣きゲー, literally "crying game") is a type of visual novel where the story is designed to have an emotional impact on the player. Most games by studio Key are nakige (e.g. Clannad). A related but distinct term is utsuge (鬱ゲー, literally "depressing game"). Unlike nakige, which usually have happy endings (though they may be only after significant hardships), utsuge are usually not as positive. The distinction is difficult to explain, but after playing a couple of games in each genre you'll definitely know. Some notable examples of utsuge would be Saya's Song or recent Key work Rewrite.
A nukige (抜きゲー, literally "masturbation game") is a type of eroge which is almost solely focused on sexual content. As the name suggests, they're intended as pornographic material. It would be unusual to describe such games as visual novels. These are the large majority of all eroge produced because they're easy to produce and still tend to sell well.
There are other terms but these are the ones that everyone should know when it comes to categorizing these sorts of games.
The four are loosely defined as follows:
These terms are quite similar but carry distinct differences, as above, and there are times when they are incorrectly substituted for one another (dating sims are often referred to incorrectly as visual novels).
