WNC is right on the money, just be aware that there are a variety of angles, 20, 17, 15, 10, 7, 5,... probably other custom angles depending on the company. And most stems are "flip flop" or reversible, meaning you can mount it up, or upside down, which explains the + / - thing. The other way a stem is advertised is in relation to being perpendicular to the steerer (90). A 5 degree stem flipped up (+) is 95, flipped down -5, is 85. Same thing with +/-17, flipped up +17, it's 107 degree's, flipped down -17, it's 73. That high school trig is finaly coming in handy eh?lol

I prefer the traditional racing style of 17 degree's since most bikes my size have 73 degree headtube's so with a 17 degree stem flipped down the stem is parallel with the ground. Makes stem mounted computers a little easier to see if you're out of the saddle.