You wire the speakers to get an ohm load that's ok for the amp. You figure out the total/final ohms of all the cabinets you have hooked up and that's what the amp will see.
For example, the amp will be working just as hard and putting out just as much power when powering one 4ohm cab as it is when powering two 8ohm cabs wired in parallel = 4ohms.
There are FAQ's and sticky's and links all over this stuff. There's probably a conversion chart somewhere that provides all the different combinations and takes the math out of the equation too.
To be more specific with my link try this.
http://www.colomar.com/Shavano/spkr_wiring_sp.html
"series" and "parallel" are wiring terms. They have nothing to do with cabs being physically standing in a series.
Most if not nearly all speaker jacks on the back of amps and plug-in jacks on the back of speakers are wired in parallel. That means + to + and - to -. Parallel connection means the ohms get lower as you string more speakers along. 8+8=4, 4+4=2. A series connection makes the ohms go higher. 4+4=8, 8+8=16. This defeats the purpose of hooking up more cabs to get louder by roughly half because higher ohms means less power from the same amp.
Wiring 2 speakers in series means you run a wire from the + side of the jack to the + on one speaker and a wire from the - side of the jack to the - of the other speaker, then run a wire connecting the remaining + and - of each speaker that's not connected to anything yet.
This would be used if you had a pair of 4ohm speakers and needed an 8ohm cab, you'd wire the 2 speakers in series inside the cab so when you plugged the amp into it, it'd see an 8ohm cab......or if you wanted to run a pair of 4ohm cabs on an amp that was not stable at 2 ohms, you'd then hook up the cabinets in series to make the amp see an 8ohm load. You'd still get a volume increase by the added speakers but you wouldn't benefit from increased volume/headroom as the amp would be running at "8ohm" power. Doubling power gets you 3db, doubling speakers gets you at least 3 and arguably 6, at least in the lower end.
This all applies to solidstate amps. Tube amps are a different animal. They either have a switch or different jacks to plug into depending on what the final ohm's are on whatever speakers(s) you're plugging into.
I can't explain it any simpler than that and probably never will.
