Doujinshis are self published mangas that are fan made. Doujinshis generally feature some popular anime or manga characters.
Like fan-made manga
Like for example a Naruto dj! would be a story created by a person (not the author) who uses the original author’s characters and writes their own story about them. Usually they’re pretty smutty tho there’s some non smut ones are out there too
Fan fiction
Oh! Got it! Thanks!
What is it? Can someone please explain? I'm new at this sorry!