Isekai
Proof that Japan has no publishing standards or quality control. Isekai is a Japanese word assimilated into the /tg/ lexicon from the weeaboo faggots at /a/ and /jp/. Literally meaning "another world" or "parallel world", it refers to a genre in which the main characters are from "our" world and taken to a foreign world resembling some form of fantasy game, where they proceed to become adventurers. Usually, plot reasons prevent them from heading home until something is taken care of - typically whatever big bad evil guy is threatening everything - but sometimes they're stuck there forever and have to adapt as best they can. Methods of transportation are vast and varied, including but not limited to: stumbling into a portal, activating a magical McGuffin, dying and being reincarnated there, being summoned by the denizens of the world, or the ever-popular getting your brain downloaded into your favorite MMORPG.
The hallmark of isekai stories is the definition of the world in terms of RPG mechanics; people in isekai worlds speak of levels, classes, and experience as real and tangible things as opposed to the mechanical abstractions fa/tg/uys normally recognize them as. Isekai protagonists tend to be big fucking nerds who immediately recognize this and exploit it, often aided by unreasonably high stats relative to their abilities in real life. The unstated implication is that the overweight slimeball watching/reading the isekai story would be just as successful as the protagonist because of his valuable and hard-earned RPG knowledge.
Although most isekai stories get panned on /tg/ for annoying meta-humor, generic shonen bullshit, generic fanservice bullshit, or a combination thereof, a handful of series are decent enough to merit genuine approval. Or tolerated because they have monstergirls, at least. Check our anime and manga pages for the current scoop.
While isekai is a distinctly Japanese form of cancer, the basic idea of people from our world getting chucked into a fantastic world and forced to fend for themselves is practically universal and turns up moderately often in Western fantasy. Oddly, when this happens it tends to be rather less shit. L. Frank Baum's Oz series, Alice's Adventures in Wonderland by Lewis Carroll, and John Carter's Barsoom novels are iconic examples of the core premise that predate cliche fantasy; a /tg/ example that (in hindsight anyway) fits the isekai mold well is GURPS' flagship fantasy setting, which revolves around people from across the universe getting isekai'd to the planet of Yrth by an extradimensional "Banestorm" and proposes that players could stat themselves and then play as themselves on Yrth after getting deposited there by the Banestorm.
It is also the source of the Truck-kun meme. As getting murdered by a drunken motorist is one of the most common ways that the typical isekai protagonist will end up in another world. Without anyone noticing or giving a shit that they are gone.