Here's a pretty good begginer's site.
I'll try to be as clear and concise, but this might be a pretty long reads. Here goes nothing:
Front Hub motors are the most user friendly, most economical and the most reliable. Friction drives(such as the Zap) can be a problem in climbing hills and if roads are wet(because they rely on a small motor in contact with your rubber wheels for power).
Rear hub motors, I have no expereince with, but it is generally accepted that they might have more power, but they can be tricky for a casual user to install, and they are more expensive, except the earlier WE rear hub motors. I don't think they sell those kinds of motors anymore.
Now, when it comes to hub motors, there's two kinds. Brushed, and brusheless motors. Brushed motors are more powerful in terms of raw power, so they have better torque and acceleration. They are better for a really hilly areas. You do need to replace the brushes every 2-5 years or so, and somebody with minimal mechanical skills might have trouble doing it. Brushless on the other hand have less torque but they are more effecient, about 25%-30% more efficient than brushed motors, and therefore have about 25%-30% more range, using the same battery pack. You also don't have to replace the brushes at all. Oh yeah, you have to pedal first to activate the brushless motor.
Now, for the batteries, most systems come with Sealed Acids, which are heavier than other more expensive stuff, but they are the most economical, and reliable. Unless you really want the more expensive kinds, are willing to install them, and have the money, then SLA batteris would suffice.
My system came with a 36v SLA(three 12 volt, 12 amp per hour(or a/h). Volt denotes power for, the higher the voltage, the more speed, and a/h denotes the capacity. Higher number means more capacity). You can change the batteries for more capacity, using three 12v, 16 a/h instead of 12v, 12a/h, for practically the same weight. My SLA sshould last about 100 to 200 charge cycles, real wolrd conditions, but even so, they are still cheaper than gas in the long run, and they sell SLA's which are capable of alot more charge cycles(1500 rated, 400+ real world conditions).
Speaking of weight, the system will not be light. Usually a battery pack(three 12v, 12 a/h) weighs 30 lbs. The hub motor will be about 18 lbs. It will be harder to pedal going uphill, but thats why you have a motor in the first place
. On flat ground, it should be ok, but you might notice it just a tiny bit when the system is new, until you get used to it. You can reduce weight by either using lower capacity batteries, or using the more expensive kinds like (NiMh, etc.), but I have no experience with those.
Range: The system I have(brushless, three 12v/12a/h batteries) is rated to have a range of 35 miles, on flat ground, 150 lbs rider, without starts and stops. Real world conditions should be around 25 miles on flat ground, with stops and starts. Of course, it depends on other variables, the same with your cars(hills, grade, tires, rider weight, cargo, etc). I'm less then 120 lbs, my bike is a heavy goliath at around 35 lbs, and I get around 15 miles, without pedalling, and some small to medium sized hills for my range. With pedalling, I should get more than 20 miles on a single charge. More if I lived on flat ground. I can also go about 18+ mph.
I'm planning on changing my batteries to have more capacity for the same wieght soon, so I should have more range.
Control: you can control the speed of the motor using a handlebar mounted throttle.
Where to get it: Check online to see if there's one available locally. I think it's better if you have first hand experience on what the kit feels like, nothing better than a physical object in front of you before deciding to splurge quite a fortune on it. I would recommend WE($350-$370) and goHub($600-$650, I've seen one on sale for under this) by largo scooters for the hub motor kits. I don't have any experience with the go-Hub kit, but from what I've seen and read, they are identical to the WE brushless kit on how they work and experience, but with better quality accesorries(battery bag, batteries and rack), and might have better customer service than WE(hated WE's), but thier kits are about $200 more(considering that they use a $40 topeak rack, and a $70 battery bag which I think is better than the WE rack and battery bag, it might not be much of a premium). They are also the only place where you can get the brushless motors since WE discontinued theirs. I have the WE brushless kit from the bargain bin for about $250 including shipping.
Zap also sells electric assist kits(not hub motors), but found them a bit more complicated and they don't fit on bikes with fenders!
Good luck with whatever decision you come upon and hope you find what system is right for you and I just burnt myself writting such a long essay. Hope that helped.