Your bio is the first time the agent you are querying is going to have a chance to know who you are. Hopefully by now, they have a great idea of your voice, from reading your hook. Now, you want to let them know about you. What you have written, published, and where. How passionate you are about your craft and why you are the perfect person to tell the story you have written.
But, what if you haven't been published before. Do you write a bio at all?
Tell the agent what you are doing to hone your writing chops. Are you part of a writing or critique group? Attended a writer's conference recently, maybe even one at which the agent himself spoke? And why are you so right for your story? Is it a medical mystery and you are an emergency room nurse? These are all important morsels that begins to give the agent you are querying a face to go along with your voice. Your affiliations with groups and clubs also reinforces that writing is much more than a hobby for you.
If you have been published, tout your street cred! All publications are important, including novels, magazine articles or pieces, newspaper pieces, and anything available on the internet. Whatever is there for the agent to find in order to get a more developed sense of your writing style.
And finally, several author friends of mine, who are also multi-published, swear that by throwing in a personal tidbit, it makes their query stand out, among a sea of other queries. I don't always do that, but I will admit to it at least once. Above all, don't fabricate. The publishing industry is incredibly incestuous and by that, I mean that everyone in the industry knows everyone else. If you make something up, someone will know. And you would hate for that to be your potential agent-goddess.
Coming next: What the Heck is a Logline, Anyway?