Picking the right airsoft battery may be tough. Nimh are the most common, typically in 9.6v and 8.4v configurations. Lipo batterys are becoming more popular. Normal airsoft guns without a mosfet can use 7.4v batteries. 11.1 Lipos should only be used with guns with mosfets. Li-Ion batteries are similar to Lipo batteries except they are smaller and last longer than lipos. The higher the voltage the faster the airsoft gun will shoot.
Mah is the milli amp that each battery has. Each Mah is comparable to 2 -3 bbs shot. Meaning that if a battery has 1000mah, you should be able to get 2000-3000rds through the airsoft gun before the battery dies