All AVR devices, like our Mega328, have ports. Usually they are a group of 8 pins that send or receive binary to or from the outside world.

There are three registers associated with each port.

1. The output register PORTX
2. The input register PINX
3. The data direction register DDRX.

For instance port B is made up of 8 pins PB0, PB1, PB2 ....PB7. It has three associated registers:

1. PORTB
2. PINB
3. DDRB

These registers live in the I/O register space just above the 32 general purpose registers.

Some good port pages:
Useful but about GCC.
Some good stuff here.