This past Saturday I gave a talk on a Writer's Platform at a small independent bookstore at the request of the writers' group that meets there. I'm up for talking about most anything. If I don't know about the topic, I can always research it.
While researching I found that a writer's platform is closely related to promotion, marketing and branding.
If you're writing non-fiction, of course the platform would relate to why you were the person (your background) to write that particular book. This could be the same for you if you're writing a book about a police officer protagonist and you are or were a police officer.
I write about law enforcement in my mysteries: In the Tempe Crabtree series, Tempe is a female resident deputy--and no, I never was; and in my Rocky Bluff P.D. series, I write about the members of a police department and no, I never was a police officer. What I wrote in my query letters way back when I had to write them was that I lived in neighborhood and was friends with many police officers and their families, I've gone on ride-alongs in a big city and small towns and I belong to the Public Safety Writers Association whose members are involved with all the different branches of law enforcement. So that could be part of my platform.
Promoting and Marketing are similar. Many publishers today ask for your promotion or marketing plan right along with your query. What is it you plan to do to promote/market your book? One acquisitions editor I know always Googles the querying author's name to see if he or she has a presence on the Web and this is part of her deciding factor as to whether she will recommend the book and author to the publisher.
Promoting might be more of what you actually do on the Web to let people know about your book: A webpage, Facebook, Twitter, other social networks, and listserves.
Marketing could also include the more in-person events you do: book signings and talks at bookstores, library appearances, speaking to local social and service groups, having a table or booth at book festivals and craft fairs, etc.
And then there's branding--isn't that also a part of what I've already been writing about? And it all boils down to name recognition. It's getting your name out there so people know you are an author and recognize your name and hopefully will want to buy your books.
There may be a better way of defining each of these terms--what do you think?