I've recently been doing some research on the work of Ellsworth Kelly, when I discovered this great article from Michael Kimmelman of The New York Times. The editorial does a good job of breaking down the meaning of a doodle and its necessary step to an artist. The following is an excerpt:
"If the doodler is an actual artist, the doodle may hasten to consciousness some qualitative thought, spark a bigger idea, which can be developed elsewhere, more formally, in directions far removed from the source. I imagine almost all artists doodle, perhaps absent-mindedly, in the way that pianists tap fingers or photographers frame pictures of what they're seeing through an imagined lens. They can't help it. Doodling is a tic of the trade."
-- Michael Kimmelman, The New York Times Decades of Doodles Help Illuminate the Creative Process