PHRF is typically used for monohull racing. Dingies and Catamarans use the Portsmouth Handicap System (DPN). PHRF is time on distance; i.e. the longer the course, the more correction the slower boats. They commonly talk in terms of "boat A gets 45 seconds per mile". Portsmouth, on the other hand, is time on time where you simply get a percentage added (or subtracted) to your finish time regardless of how far you sailed. Porsmouth also has the ability to factor in wind strength whereas PHRF does not (to my limited knowledge anyway). Handicap Racing time on distance has never made sense to me.

For more details (including calculation examples) go here: http://www.ussailing.org/portsmouth/

for PHRF go here:http://www.ussailing.org/PHRF/