Both freezers and fridges work on a 'refrigeration cycle'
http://en.wikipedia.org/wiki/Refrigeration_cycle.
A working fluid (refrigirant) vapour is compressed so it heats up, it then passes through an condenser to cool it down to a liquid. This is then throttled, it's
pressure is lowered, which drops the temperature. It's then passed through the fridge to cool the air inside.
The key to why a freezer is colder than a refrigirator is that the working fluid is throttled through an expansion valve to a lower temperature.
http://www.howstuffworks.com/refrigerator4.htm
A summary. The fuild inside the fridge freezer plumbing cools when expanded. In the freezer it is expanded more to allow it to drop to a lower temperature.
In combined fridge freezers that run off 1 system. Two throttles are used, one that goes to the fridge and the other goes to the freeezer.
