I have read through the reference material about the ports: http://www.arduino.cc/en/Reference/PortManipulation and it made sense. It reinforced what I see in the spi.h file when it references PORTB and that port's associated bit registers.
What I don't understand is their connection with the hardware registers like SPDR and SPCR. In other words, if I am using two separate ports' registers and they both are using the system registers, how is all that kept straight? If there is a link someone could point me at, that would be great.