Excerpt from this article:
Part of the Internet’s beauty is the space it affords people to take an idea and run with it . . . and run with it and run with it, until they end up miles away from where they started. Among a subset of online superfans of the BBC show “Sherlock,” solving mysteries is mere window dressing for the real story: one about unrequited love between Sherlock Holmes (Benedict Cumberbatch) and Dr. John Watson (Martin Freeman).
This sort of wholesale invention, by viewers, of a romance between fictional characters who are not romantically linked on-screen is a form of something known as “shipping” (short for “relationship-ing,” the term can also refer to rooting for actual fictional couples).
It is by no means limited to “Sherlock” — any form of pop culture, from “Scandal” to One Direction, is fair game — but that show has inspired vast and vivid fictional worlds, completely imagined by shippers who share screenshots, drawings and even entire books…