Voltage is just voltage - meaning how much power is being fed to the processor. Lower voltage kernels will have a positive effect on battery life and will run at lower temperatures. The voltage is set by the kernel - usually labeled standard/medium, low, or ultra low. However, when overclocking any processor, turning up the speeds may require higher voltages. Even though we're all using the same phone, not all processors are created equal. Some of them may accept a speed of 1200mHz with no problem at low or ultra-low voltages. Others may require a medium voltage kernel to be stable at that same speed. You have to try some and see which works best for your Droid.
Generally you should start experimenting with the lowest voltage kernel possible and gradually increase the speed in SetCPU if you have it, testing the phone for at least a half hour (IMO) to make sure it does not reboot or freeze during use. If it's stable, you can go higher. If it's not stable but you want to use a certain speed, try a higher voltage kernel.
As for the slots, that just refers to how many "steps" there are between the minimum and maximum speeds set by the kernel. For instance, a 1200MHz, 5-slot kernel may have speeds of 250/400/600/900/1200. This is just an example, but basically those are the only speeds you can run the processor at. A 7-slot kernel may have two extra ones thrown in there, so you have more choices, but they have been said to have decreased stability.
