PDT vs PST
Pacific Time Zone is observed in the US and Canada and is generally called Pacific Time or PT. Since this time zone is based on 120th meridian west of Greenwich, the local time is calculated by subtracting 8 from GMT (UTC-8). This time zone is called Pacific Standard Time (PST) in winters (when observing standard time), and Pacific Daylight Time (PDT) in summers (when observing daylight saving time).
Standard time is regular time which is either ahead or back of GMT. Daylight time is when clocks are moved an hour forward in an attempt to save time. To make maximum utilization of sunlight this step is taken to move the clock forward. It is when summers are over and it is fall time that clocks are again turned back by an hour. Thus PST is Pacific Standard Time and is observed when we turn back our clocks by an hour, and PDT is Pacific Daylight (Savings) Time when we turn our clocks forward by an hour.
Daylight savings Time comes into effect every year on second Sunday of March at 2 AM local time and continues till November when it is again pegged back by an hour to revert back to Standard Time.
The states in the US where PDT is used include California, Idaho, Nevada, Oregon, and Washington. Canadian provinces where PDT is used during summers are British Columbia and Yukon.