A "NSFW" comic is basically determined by a few simple factors:
1.) Is it something that would be safe to view in the workplace? (things that you would not want to be caught looking at while at work can include nudity, extreme language, and sensitive subjects).
2.) Is it something that you would be fine showing your mom?
Most NSFW comics have adult content like nudity, extreme swearing (and I mean vulgar language up the wazoo lol), sexual/pornographic content, suggestive themes, etc. If it's something you think that someone could get fired for accessing and looking at, it's considered "Not Safe For Work".