Can some one try to make sence of the hundreds of mAh ratings on batteries? what do they mean. How do I know which motor takes which mAh?

MAH (milliamper hours) is similar to "how big is the tank", the more milliamper hours, the longer you can drain a fixed current from it.

So as long as the volts are correct for the motor i can use larger or smaller mAh ? will a charger for 300 mAh charge a 400 mAh

Yes & yes

Kev, what model and motor do you wanna power? I have around 30 electric planes from the small puny J250 motors up to large brushless types. I also use nimh, nicad and lipoly battery packs.
Heres an example of mah and how it figures into flight time......
A 6 volt speed 400 motor with 5.5x4 prop and 8 cell battery pack will roughly consume 10 amps. So, lets say you have a 2000 mah 8 cell nimh pack. So, in 1 hour, you will consume 10 amps (or, 10,000 amps). In 1/2 hour, 5 amps and in 15 minutes, 2.5 amps. So, using this 2000 mah pack, at full throttle, you will have around 12 minutes or so run time.
As far as chargers go. Doesn't matter if the charger charges at 50 milliamps or 5000 milliamps, as long as the correct voltage is set.
Remember, each nicad or nimh battery cell is 1.2 volts. For lipoly batteries, its 3.7 volts per cell. Most battery packs will have a C rating for charging and discharging. Nimh packs, you should charge them at roughly 1C. So, if you have a 1600 milliamp pack, charge it at 1.6 amps. You could even go to 2 amps but I wouldn't push it. Nicads are different in that they can take fast charging.
If you have a 2100 milliamp SCR pack, you could probably charge them at 5 amps.
Lipoly cells are different. You MUST set the correct voltage or risk fire and explosion. They are very volatile if not taken care of during the charge. You always charge them at 1C. They are expensive but, provide more "punch" and their discharge is more linear than nicads or nimh's which tend to start sagging some even in the middle of a motor run. Nice thing about lipoly's, they weigh 50% or more less than nicads and nimh's. Makes them ideal for the smaller brushless powered 3D ships out now.

Dave...