grraphal,
First of all, welcome to RCU. Hopefully we can help you get up and running in this great hobby.

They list the specs of servos at two different voltages because you can run your receiver setup in the plane at either voltage, either 4.8 volts or 6 volts. Each setting has advantages and disadvantages. For 6 volts your get more power and quicker movement out of your servos, but you batteries don't last as long. For 4.8 volts you get better battery life, but you don't get as much power or the servos don't move as fast. In the context of beginner's planes almost all of the radios that you will see will run at 4.8 volts. Most people don't switch over to 6 volt until they get into bigger planes.

Dual rates is a term used for your radio which determines how far the servo will move on a given control surface. At a normal setting a control surface will move a given amount of travel. But let's say that we don't want that surface to move so far, we will set our dual rate as a percentage of the full rate. So for instance we choose 50% on the dual rate, when it's turned on the control surface will move only half as far as it will on full rate. These are usually used when you need to a control be less sensitive for certain situations. One common use that is a good example is for elevator control. When in normal flight the elevator will be kept at full rate so that you have as much control over the plane as possible, but when it's time to land having that much throw is a problem because it causes the plane to jump around when you're applying elevator corrections. At this point the dual rate (or low rate) will be used so that the elevator becomes less sensitive and is easier to control for a landing.

Hope this helps

Ken