It varies from person to person and company to company; there is not a standardized definition, in the publishing industry, for these terms, as much as readers and authors would like there to be one. A good portion of learning what fits a line...or which lines match your personal ideas about the terms...will come in the form of research/trial and error. When a company has a rating system, you might want to study it and start applying it to your own sensibilities.
Some companies are confusing the issue further by having lines that, for instance, state on the spine that they are erotic romance, state on their guidelines they are erotica and (in reality) accept both. This is not a good situation for readers, IMO...and there are plenty of blog posts complaining about it, as well. You see, it's not that the majority of readers of erotic romance would be offended by erotica, though some might be, since they prefer the HEA of a romance genre, which is not guaranteed in erotica. The problem is that they choose something that is REPRESENTED as what they are searching for, buy it and discover it's a bait and switch. Who would be happy about that? It is a disservice to the reader, the book and the author.
Back to the generally-accepted definitions, in most of the publishers I deal with. There are others, but we'll focus on the definitions from many indie/e-publishers. Independent press is where these genres first gained their popularity and audience. NY is the newcomer to the game, so their confusion of the issue and skewed definitions are not my primary concern. Since none of them can agree...and many are mislabeling by mixing genres under the same definition, addressing the NY companies would be, largely, like beating your head against a brick wall.
So, what is the difference? FOCUS is the important thing...and content to a lesser extent.
Sensual Romance- Take a traditional romance that doesn't fade to black. There is sexual tension between the characters, who definitely DO consummate the relationship, at some point in the book (unlike traditional romance, which may leave the characters, before that step occurs). In a sensual romance BOOK, it is expected that consummation will occur. In a sensual romance short story, it may not. Just thought I'd make that distinction clear. Sensual books, by definition, engage the senses of the reader. You have a moderate amount of detail in the sex scene and not amorphous emotional responses to unknown stimuli, as you find in some romances. In addition, such books may contain a bit of mild BDSM/bondage play, toys, etc. Sensual romance may also include more than one sexual interest for the main character, sometimes both realized sexual partners at some point in the book, usually not consecutively in sensual books. (Think of the woman who leaves a bad relationship and enters another. Or...a woman who has two sexual interests and settles on one, but usually not sleeping with both, if it's sensual.) As stated before, the development of the romance is the central (or in the case of cross-genre, co-central) plotline. I disagree with the RWA (as per the Passionate Ink definitions) that the scenes can necessarily be removed and still have a strong book. Since all sex scenes should advance characterization and/or plot, deleting sex scenes should (theoretically) make the book weaker. HOWEVER, I do agree that the scenes in a sensual romance can often be toned down. I've recently done this, and I don't think I lost much of anything, in the bargain, because I lost detail...not emotion, not characterization, not plot. Sensual romance MUST include a HEA, unless you are writing a sensual dark romance.
Erotic Romance- Erotic romance MAY include (but does not necessarily include in any given book): more frequent sex scenes than a sensual romance, more detailed sex scenes, multiple sexual partners (in the book or even at the same time...in fact, poly relationships are perfectly fine in erotic romance), harsher language/coarser language, extreme sexual play/BDSM...as long as it's consensual (it MUST be safe, sane and consensual), a more intense sexual/sensual experience. (In an erotic romance BOOK, it is expected that consummation will occur. In an erotic romance short story, it may not. Just thought I'd make that distinction clear.) An erotic romance MUST include a HEA, unless you are writing erotic dark romance. Contrary to what the RWA says, an erotic romance certainly CAN explore the sexual journey/discoveries of the individual and how that affects the individual, which they reserve for erotica. It may also explore the sexual mores and how they affect sexuality or how sexuality challenges them. This is one of the MANY reasons I think the RWA definition falls short of the reality of the offerings out there already. They are too narrow, by my estimate.
Erotica- Erotica does not have the requirement of a HEA. It does not have the requirement of a romantic relationship. In fact, many erotica stories are about f**k-buddies, mistresses, BDSM trainers, one-night-stands...even complete strangers. Erotica is sex for the sake of sex and what the individuals learn/experience, not always with a mind to the repercussions of said acts. It might be about the sexual discovery, the sexual journey, the challenge of sexual mores and expectations... It might simply be someone with a need to experience, to break out, etc. It depends on the needs of the plot and characters. Again, the sex scenes should serve a purpose. They should advance characterization and/or plot. They should be safe, sane and consensual. A modicum of respect between the characters is my personal rule, but some "erotica publishers" don't expect it. I do.
Now, let me go back a moment. Erotica doesn't require a romance, but if it had one, some people would assume that would make it erotic romance. Wrong. Why? The proof is in the focus. Is the focus ON the relationship, as explored through sexuality? You have erotic romance. Is the focus on the sexual discovery, from which a romantic involvement evolves? You have erotica.
Porn- When you leave SSC behind...or respect...or sex scenes that serve a purpose and advance plot/characterization... When you sacrifice plot and characterization to "stroke fiction"... At that point, you delve into my personal definition of porn. You don't have to do all of them to accomplish that switch.