[Patrick]

Paralel adds the capacity of the batteries, series adds the voltage.
Two 3.7v 4000mah cells wired in paralel would be a 3.7v 8000mah battery.
Two 3.7v 4000mah cells wired in series would be a 7.4v 4000mah battery.

Using cells in paralel divides the current draw between them, making it easier on the cells than if they had to put out the full amount of current by themselves.
Some batteries are wired in paralel and in series.