At its broadest, my general audience is teenagers. Throw in adults who like smart YA work, too, and there you go. To get a little more specific, I think my novel appeals especially to girls who like math or science, and/or have a bit of tomboy in them.
While at Best Buy with my sister yesterday, I found a succinct description of the sub-population who would most enjoy my work:
Girls who bought StarCraft II
That's why we were there—for my sister to buy the brand-new game. The cashier (female) asked if my sister was buying it for herself. When we confirmed she was, the cashier said, "Finally! Another girl that plays!"
Those girls (or the ones that play their brother's/boyfriend's copy) are exactly the ones I wrote the story for. It turns out other groups of people enjoy it, too, which thrills me. But they're the ones I was aiming for. That's my Audience ID, the quirk that puts them on just the right wavelength.
What's your Audience ID? Something that on the surface may seem to have nothing to do with who they are as a "reader" but describes who they are in under five words. Kind of tricky until you run into the right cashier.