Nicad, Nimh, or Lipo is just the chemicals on the inside used to create the eletricity...each type of battery reacts a little different to the load applied to it...basically how much and how quick of a voltage drop under a load you'll have.....Each are far better than carbon zinc batteries like A, AA, C, etc..
for glow or gas powered aircraft most common voltages are 4.8 and 6 volt.... 6 volt will make your servos more powerful and quicker..so why not use 6 volt at all times?....because enough torque on a servo or quicker servo is not always a good thing..and frankly enough is well enough ...if you know what I mean..
The milli amp hour mah rating is simple...1100 mah battery can produce 1100ma for 1 hour or 1ma for 1100 hours 650 ma for 2 hours etc etc..
Most flight packs come with a 4.8 v 600 mah battery..that is enough with 4 or 5 analog servos to fly 3 or 4, 15 minute or so flights on a full charge....