This is a question I started to ask myself as I scrolled through prompts on Tumblr, Instagram, Twitter, Reddit, and whatever Google had for me. There seem to be in general three different kinds of prompts. Note that these are extremely broad categories, and there are probably more I haven't thought of.
Behind Door #1 we have Nonfiction Prompts.
What it is: These are what you'll usually find in those "150 things to journal about" lists, since they assume you want to write about things that actually happened to you, or people you know in real life.
How to do it: I find that nonfiction prompts are most effective when they call on strong memories without digging for a specific situation. Things like "Remember a time when a gift you gave made someone really happy," or "Who in your family made the best breakfasts?" They're almost always questions, since they're trying to dredge up memories, rather than inspire fictional situations. Unsurprisingly, they're also the easiest to work with in a classroom setting, as they're most likely to get you relatively similar results with a group.
What can go wrong: These prompts can sometimes come out as "cookie-cutter" or otherwise unoriginal, calling on similar situations or feelings repeatedly.
If we pull aside the curtain for Door #2, we'll find Suggestion Prompts.
What it is: As the first of our fiction categories, I think you can probably remember a few you've seen, either online or in a creative writing guidebook at some point. These are the easiest to come up with off the top of your head, but also the harder of the two fiction options to pull off well.
How to do it: The best format I've found for suggestion prompts is to follow a basic formula - [Object/person + remarkable quality + situation]
The more unusual or out of context the situation seems, the better. As a rule of thumb, suggestion prompts tend to work like jokes, by the Rule of the Ridiculous. You'll notice, I think, that the punchline of a joke is always funnier if it's surprising. A child tells you a joke about a banana in the swimming pool, and it's hilarious to the child because the banana is in the swimming pool. It's ridiculous that a banana would be in that context. Suggestion prompts are the same way - the object or person is not in its usual context, and that makes it interesting, frightening, or funny. It's the emotion the prompt inspires that will drive a reply.
What can go wrong: The main problem with suggestion prompts is when the person or object used fits too well inside the context of the situation, or the remarkable quality is too unbelievable for the suggestion to be coherent.
Finally, if we pull down the ladder for Door #3, we can see Narrative Prompts.
What it is: As you might be able to tell from the name, these are the prompts that contain a description, a piece of dialogue, or some exposition that might have come out of a story. The goal, of course, is to inspire writers to continue the scene that you started.
How to do it: These prompts are relatively easy to pull off well, simply because they are more difficult to set up. Making the description, exposition, or dialogue coherent is more of a challenge than placing a person or object into a silly or frightening or strange situation.
What can go wrong: These prompts do, however, run the risk of including too much detail as the writer providing the prompt has too specific of an idea in mind for what he or she wants the result to look like. This is what we call "railroading," and generally comes of a writer not wanting to give up the control of an idea to other writers.
Those are the three categories of creative writing prompts that come to mind today, but do let me know if there's one I missed. I'm far from infallible, and sometimes another pair of eyes can teach me something new!