If you cut and paste or link against GPL'd code into your application, your application must be licensed under GPL and you are then required to release the code.
However, you can still sell your application and afaik, the only oblication is that you release the sourcecode to your customers.
If the library you link against is Lesser Gnu Public License aka LGPL then you dont need to release your own application's code but you are still required to release all modifications if you modified the lgpl'd code.
