One of the things we studied at McDaniel College was the difference between a premise and a story.
A premise is an idea. Dictonary.com defines it as "a proposition supporting or helping to support a conclusion." It comes from two Latin words meaning "to put before." So, basically, it's the underlying idea that supports your story--and has to come before you can build your story.
A story, according to Aristotle, has a beginning, a middle and an end. Stories also have characters, settings and plots.
Your premise asks the question, "What if...?" Your story answers that question.
Robert Louis Stevens once drew a map to pass the time on a rainy vacation. The map
inspired Treasure Island.
The Lion, the Witch and the Wardrobe came to 16-year-old C.S. Lewis as a daydream.
Jules Verne got the idea for Around the World in 80 Days from a newspaper advertisement offering such a trip.
A lot of my writer friends complain that it's tough to come up with ideas, but once they have an idea they can run a marathon with it.
I'm just the opposite. Premises grow like weeds in my brain (probably thrive in all that manure). It's story that's tough for me. Figuring out what kinds of characters and plot will allow my premises to blossom into full-blown stories is like trying to make fire from flint. If all I had to do was strike rocks together and generate sparks, it would be great. But there's that whole mess with tinder and twigs and small branches and making sure there's enough oxygen and.... Okay, that metaphor's getting away from me.