Update, Feb. 11 at 7:30 p.m.: Initial results of the Maine Straw Poll are in. See the latest post, Romney squeaks by Paul...
In late January, Maine Republicans began making their choice for a Republican nominee to face President Barack Obama.
Saturday, Feb. 11, is the final day for voting in most counties. The Maine GOP said Washington County has been postponed until Feb. 18, however. The Maine Republican Party website provides full details about that.
As best I can tell, this caucus works like others that are non-binding—people select their presidential preference in what amounts to a straw poll. There’s a lengthy explanation of the process on the Maine GOP page linked above.
The Atlantic shed some light on what amounts to a complex situation for a small state like Maine: