PST and PDT are both time zones. Both are used in the United States, Canada, and many other countries. PST stands for Pacific Standard Time. PDT stands for Pacific Daylight Time. PST is used during the wintertime, where the PDT is used during the summertime.
This is where Daylight's saving time comes into play. When PDT is in use, time is moved an hour up. When PST is in use, it is used an hour behind. This is where the phrase "Fall back and spring forward" comes into play. During times of the year when it is warmer, we will sprint forward for the purpose of using more sunlight and conserving energy.