NSFW means Not Safe For Work. It is sometimes rendered NWS.
NSFW also includes schools, naturally enough.
If something is NSFW, you should clearly label this fact, because people tend to not want to see things that are NSFW while they are at work or school because it can get them into trouble. Any link that leads directly to something that is NSFW should be marked.
SFW, on the other hand, means that it is indeed Safe for Work.
NSFW's close friend is NSFB.