I googled a pic and this seems to explain it pretty well. Figure A is in series and Figure B is in parallel. Off of a 12v supply you would want to hook all your fans in parallel. This means that all the fans hook directly to the battery/adaptor and not to each other. If you wired two fans in series they would split the voltage giving each fan roughly 6 volts. Running in parallel uses more amps and will also drain your batteries faster.