When batteries are joined from positive to positive, and negative to negative, this is called parallel. Think of it as a two fences. One for the negatives, one for the positives. They are parallel to each other and never cross over each other.
Parallel connections increase the capacity, and keep the voltage the same. If you hooked up two 6 volt 10 amp hour batteries this way, you would get 20 amp hours at 6 volts.
Why would you want to use parallel? To increase the time you can have the power on at the same voltage.
When batteries are joined from positive to negative, this is called series. It only takes one positive and one negative from each battery to be joined together, not both sets like in parallel.
Series connections increase the voltage, and keep the capacity the same. If you hooked up two 6 volt 10 amp hour batteries this way, you would get 10 amp hours at 12 volts.
Why would you want to use series? To increase the power you have for that time by increasing the voltage.
There are a variety of reasons you would want to wire either way, or both ways at the same time. We can help you go over the pro's and con's and the why's and why not's for each method.