This page: http://www.arduino.cc/en/Reference/PortManipulation
Has a section that includes:
//See the bitwise operators reference pages and The Bitmath Tutorial in the Playground
PORTB is the register for the state of the outputs. For example;
PORTD = B10101000; // sets digital pins 7,5,3 HIGH
You will only see 5 volts on these pins however if the pins have been set as outputs using the DDRD register or with pinMode().
PINB is the input register variable It will read all of the digital input pins at the same time.
It might be more clear if the PORTB reference was changed to PORTD and PINB was changed to PIND.