In short, as far as porn is concerned, if it's a movie with a plot, it's called a "feature". If it's a movie without a real plot throughout, but several short plots or stories to accompany each sex scene, it's called "vignette" porn. If it's a bunch of sex that's just filmed and nothing else happens, no story, no conversation, no interaction with the camera person or anyone else, etc. it's called "wall to wall". Everything else is "gonzo".
Further, the "wall to wall" and "vignette" flicks get thrown under the "gonzo" umbrella most of the time.
These characterizations have far less to do with the type of sex that's occurring than they do with the type of movie. But at the same time, it so happens that the majority of the people filming features are aiming their stuff at the "couples" market, so they're the ones who usually film the boring, vanilla, ma-and-pa sex. There are exceptions, of course, but in general that's how it goes.
If you're seeing sex that is at all unusual, it's probably in a gonzo scene, with the exception of Eli (and maybe the odd other director, like with John Stagliano's Fashionistas series). There may be the odd Wicked or Vivid or Digital Playground release that pushes the envelope, but for the most part that's not how it goes.
So, for example, the entire kink.com family of websites would all be characterized as "gonzo".
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