NSFW
NSFW means 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. Everything else is perfectly work-safe. As a result, even 4chan's supposedly "work-safe" boards are plastered with pictures that would give your grandmother a heart attack faster than forcefeeding her an entire loaf of meatbread.
On 4chan, work-safe boards have a blue background by default, whereas NSFW boards use the standard futaba colour scheme.