Sorta like Qwk said
Series ADDS(not doubles) voltages, but keeps the battery capacity the same. If you wire a 2S, and a 3S battery it will be a 5S battery (Keep the current the same) this is the same with NiMh, NiCd, and all other votages.
Parallel adds the batteries capacities together, but keeps the voltages the same. In this case again would want to make sure the capacity, and specs for the cells are the same so in this case you can say it is multiplicative. So if you have a 5000mah battery you would want to do another 5000mah battery(you can technically add as many as you want) which would make your capacity 10k mah.
You want to match the cells capacity, and current draw specs so you don't over draw a cell, and damage one which could lead to failure.