All standard batteries provide this voltage. 1.2V, so an 8.4V battery uses 7 cells. But be warned a higher voltage will increase wear and tear significantly on un-modified guns. E.g. a 1400 mAh battery will supply 1400 mA for 1 hour. The higher the number the longer the durability.
Larger cells have a higher capacity simple.
Buying a battery with a higher capacity has no adverse effects on your AEG it will just power it for longer. So my general rule is to take 2 large batteries into the field, and when I am using smaller packs take 3 packs into the field. The only one of real significance to airsofters at the moment is NiCads. NiMH batteries are more recent and offer a higher peak current, but as they discharge the current continually decreases, so your RoF will continually decrease. Basically stick with NiCads since they are what AEG s are designed to operate with, anything else could harm the gun or just not work.