Hey Mate,
Ohms is a measure of resistance. In relation to car audio the "resistance" (also known as the impedance or load) of a speakers voice coil is measured in ohms, a car audio speaker is usually rated at 4ohms.
Now, when you wire a speaker to an amplifier the amplifier will "see" a resistance determined by the speaker and/or wiring configuration and will output a certain amount of power/watts, for example a subwoofer with a single 4ohm voice coil wired to a mono-bloc amplifier will present the amplifier with a 4ohm load.
The load on an amplifier (and therefore output) is not only determined by the voice coils impedance but also how it is wired up. E.g. If you have two subwoofers with single 4ohm voice coils and you wire these in parallel to a mono-bloc amplifier the resistance effectively halves, presenting the amplifier with a 2ohm load which will affect the output of the amplifier (usually outputting about double compared to a 4ohm load). If you wire the subs coils in series this will double the resistance and therefore half the output of the amplifier.
The power handling, impedance and whether or not the sub is a single voice coil (SVC) or dual voice coil (DVC) will need to be matched to an amplifiers power output and impedance, otherwise you risk frying an amplifier or blowing a sub. It is best to stay within 10% of a subwoofers power handling for optimum results.
In regards to running your amplifier at either 4ohm load or a 2ohm load, the lower the resistance the more heat that is produced by the amplifier and the distortion increases. That is why audiophiles that are obsessed with sound quality prefer running their speakers at a higher impedance. It is better for the amplifier to run it at a higher impedance and it is incredibly rare to find an amplifier that is able to handle less than a 1ohm load. When you are dealing with subs the distortion caused is hardly noticeable and unless you go below the impedance handling of the amplifier it is perfectly acceptable to run them at a lower impedance.
The reason alpine offers both a 4ohm and a 2ohm variety of their subwoofers (I'm assuming they are both DVC) is that each one can be wired to achieve a different impedance. For example: Let's just say your amplifier outputs 400wrms @ 4ohms and 800wrms @ 2ohms. The subwoofer you are looking at is a Dual Voice Coil, has a power handling of 400wrms and is offered in either 2ohm or 4ohm voice coils. So to output 400wrms the amplifier must "see" a load of 4ohms. In this case the best choice would be the 2ohm DVC variety as you can wire the coils in series (doubling the resistance to 4ohms) and wire it to your amplifier. Perfect match!
Understanding ohms and how they are used in car audio can be tricky to understand at first but once you get used to it it will become second nature.
Good luck,
Ben.
ps. Sorry about the length, I tried to explain it as basic and straight-forward as I could.
