|This article contains PROMOTIONS! Don't say we didn't warn you.|
Now Show Friends and Workmates Not Safe For Work. On the majority of the Internet, this means images that are even somewhat risque. 4chan, on the other hand, operates under a much more specific definition of NSFW, by which only the presence of nipples or genitals make an image NSFW (although this is not actually an official ruling, it is how the NSFW rules are generally understood and enforced). Everything else (including text) is perfectly work-safe. As a result, even 4chan's supposedly "work-safe" boards are plastered with pictures and stories that would give your grandmother a heart attack faster than forcefeeding her an entire loaf of meatbread. And even on those, nipples and genitals make regular appearances, to the joyous cry of "PROMOTIONS!"
While it's easy to tell if something humanoid is NSFW or not, occasionally something bizarre will get posted that is probably NSFW but you can't be sure, because the subject of the image or text is not human.
On 4chan, work-safe boards have a blue design (known as "burichan") by default, whereas NSFW boards use the standard red/yellow "futaba" colour scheme.
A related term that's rarely used is NSFL, or "Not Safe For Life." Usually reserved for things so shocking or bizarre that it will melt your mind.