The terms Beam Angle and Field Angle are two commonly used terms for describing a lights output. Knowing what those terms mean is important when you are selecting a light that's right for your application.
To understand what they mean you should first understand that when a manufacturer measures and advertises a lights candlepower they are talking about the intensity of the light source in a specific direction. In most cases this direction is directly in front of the bulb and is taken at the center of the beam the intensity is the greatest.
Beam angle is the smaller of the two numbers and references the portion of the lamp's beam where the candlepower is greater than 50 percent of the candlepower measured at the center of the beam.
The Field angle is the larger number and it measures the area of the lights output where the candlepower is greater than 10 percent of the center beam candlepower.
The image below is meant to serve as a reference so that you can see how the beam angle effects the beam pattern farther from the source.
The first thing you have to determine when trying to decide which lights are right for you is what your goals are. Are you looking to see farther down the road, are you looking for better lighting in poor driving conditions like rain and fog, or are you looking to do both?
If you answered that you want to see farther down the road, especially at highway speeds, then driving lights are going to be the style of light that you are looking at. A driving light is meant as a compliment to your bikes high-beam and has a more focused beam with the goal of casting as much light, as far down the road as possible. Driving lights beam angles tend to be smaller so that they can project farther down the road, making you aware of hazards sooner.
Fog lights on the other hand are intended to compliment your bikes low beams and provide more light closer to the bike. Their design is such that the light output has a wider beam angle and is focused down at the road, having a clear cutoff point so as not to project light up very high. The reason for that is that, the higher the beam is projected from the light, the greater reflection you get from the light in rain and fog.
If your goal is to have more light projected farther down the road for advanced hazard warning AND to greater visibility in poor driving conditions then the ideal situation would be to install two sets of lights on your bike. The first set would be the driving lights and these should be mounted as high as possible to project the light farther down the road. The second set would be your fog lights and these should be mounted approximately 10-14" above the ground for the best performance.
No matter what lighting you choose for your bike you should always take the time to make sure they are adjusted properly. Poorly adjusted lights aren't just an annoyance to oncoming traffic, they are a safety hazard.