Ported enclosures use a tuned port to get a little additional output. Typically from the back side of the woofer. The woofer's particular characteristics, along with the size of the box and the size and shape of the port determine at what frequency the extra output occurs. Viewed on a graph, the boost in output is not usually a sharp spike but rather a rounded bump centered on the "tuning frequency". Once you get much below that frequency, the output drops off drastically and the woofer is out of its controlled range. I like deep bass, which is why I typically tune to 30 or maybe 40 Hz, which is easy to do with JL Audio subs.
As far as "how you tune", it's really nothing more than knowing the parameters of the woofer, then sizing the box and ports according to where you want it tuned. The easier (and recommended) method is to use the charts provided by the woofer manufacturer. Just build it right and it comes out tuned right. The "tuning" is in the construction of the box. There's nothing to do after that.
I'd recommend getting with the guys in the car audio subforum, like chuyler and gotwake. They're the experts on this stuff and can help you a bunch.
