In a nutshell, yiff is furry porn.
About 99% is produced by the Furry Fandom, who invented the word (which makes sense). WikiFur seems pretty confident that some furry fan named Foxen invented the word, as part of a language called Foxish.
Foxish was an entire made-up language consisting of a grand total of nine words used for fantasy role-playing foxes. Yiff had the original meaning of doubleplusgood, apparently. The word comes from the onomatopoetic rendering of the sound that a happy fox makes. The word became a catch-all for sex acts in the furry fandom somewhere along the line, though it also became a classification of artwork used to specifically refer to any artwork of both an anthropomorphic and adult nature.
Of course, yiff has its critics. For a lot of people, yiff skews just a bit too close to bestiality due to the animal-like characters pictured. This can be countered by the observation that the characters are anthropomorphic, or human-like, adult animals, which implies that consent can be given. Plus the fact that most furry characters are essentially human from the neck down (including all the... relevant parts... Most of the time). It should also be noted that while cartoon porn is considered a fetish known as schediaphilia, there is currently no distinction between more common cartoon porn fare and yiff art within fetish classifications.
It is also unfair to say the Furry Fandom has produced all yiff; poor Mickey Mouse has starred in pornographic Tijuana Bibles, along with other Funny Animal characters, making this older than you might think.
Note that the actual term yiff is largely a Discredited Meme at this point; it might occasionally be used in private conversations, but expect to get laughed or derided out of the virtual room (by furries, no less, which is saying something) if you use it in a serious — or sexual — context.
No examples, please.