What is it?
It's a place to stand that can be jumped off of to take a nose-dive into the abyss ...or it can a springboard to launch into the air.
I have a solid platform that is flexible enough to send me where I want to go:)
A platform is the term of art that explains everything you can do to market and promote yourself and your products.
That differs from a business plan, how?
It is a business plan in many respects, except it's called AUTHOR'S PLATFORM in the context of publishing.
Jeff, Have you written a "Guide to Platform Building"? If not, do you have a favorite reference for us?
How do you know if your platform is strong or flexible enough to stand up to marketing a book?
It comes down to how many people you can reach who care about what you say and do.