What are the default configurations? What custom configurations are available?
For the default configurations of each model, please refer to the specs pages for the model of interest. In addition to the stock configurations, you also can — or could — choose to customize the processor (in some configurations), the amount of RAM, the hard drive, and whether or not to add a display adapter, an external Apple USB Modem (no longer available), or a MagSafe Airline Adapter. Other options have been offered for different models at different times as well.