NSFW is an abbreviation for Not Safe For Work. It is used to designate the sort of thing which gets you fired if your boss catches you browsing it. For most people, this is any pictures, video, or audio above PG-13 rating; it is not necessary to tag pages with plenty of swears, though some cunt-eating dog fuckers do so anyway.
Failing to tag a NSFW link as such in IRC is an offense for which kicks and sometimes bans are just punishment.
Tagging a link NSFW when there is absolutely no nudity involved is rather mean and disappointing. If you'd like a full list of NSFW material on this wiki, see the appropriate category.