Acceptance Angle of FM Antenna

I am going to buy a roof mounted FM antenna. I notice some models have different acceptance angles. What is that? Would it be best to use a rotor with the antenna or just aim it were the signal is coming from?
The "acceptance angle", as I understand it, refers to the orientation of the antenna that produces the strongest signal, give or take some level / db. It is sort of inverse to the "rejection angle", where the antenna produces the weakest signal or null. Ideally, a directional antenna (such as a Yagi) will have a fairly narrow angle of acceptance, so it delivers a lot of clean signal and very little spurious noise or distortion. On the other hand, an omni-directional antenna will receive signal in a broad area, even up to 360 degrees. If you are using a directional antenna, it makes good sense to mount it with a rotor. That way, assuming your tuner has a good signal meter, you can aim the antenna directly at the strongest signals in your reception area.
Sounds like this "acceptance angle" is inversely proportional to antenna gain. You want that gain, normally the more the better. If you want to listen to stations from different cities, then a rotor is the way to go. If you're only focused on 1 city then just aim the antenna using your tuner's signal strength meter as a guide.
Above answers are correct. The real question you need to answer is how far appart in angular extent are your favorit stations. The 3db down point either side of the antenna's main axis is refered to by non engineers as the acceptance angle. This means if you slew the antenna that many degrees off the main axis (bore site on the station's transmit antenna)you lose 50 percent of the power at your receiver as delivered by the antenna. So if you have an acceptance angle of 5 degrees (a fairly high gain antenna) and your two favorit stations are 20 degrees appart, there is no way you can receive both stations properly without a rotor to move the axis of the antenna from one direction to the other. On the other hand if you live very close to your stations get an omin directional antenna with a gain of 1 and an acceptance angle of 360 degrees.