The Pacific Crest Trail (PCT), like it's east coast sister the Appalachian Trail (AT) is a continuous hiking trail running from the southern terminus at the Mexico-United States border to the Canadian border in the north. It is 2663 miles long (approximately 500 miles longer than the AT) and passes through 7 national parks. Unlike the AT, which tends to run almost exclusively through forested areas (it is nicknamed "The Green Tunnel"), the PCT passes through extremely varied terrain including desert, the high sierra mountain range, the wet forested area through Oregon, and the volcanic cascade mountain range in Washington.
The PCT is also more remote than the AT, with fewer road crossings and towns and longer stretches of wilderness. (No mid Atlantic corridor with twice a day trips to delis!-- I'm talking to you, New Jersey.) It makes for more specific planning of mail drops and more difficulty with navigation. Whereas on the AT, all you have to do is look for the white blazes, on the PCT is unmarked. You actually have to know how to read a map and use a compass (or at least a GPS and a navigation app). Plus, there are often re-routes on the PCT, depending on things like snowpack, wild fires, or unsafe terrain.
Comparing the elevation gain and loss between the two creates an interesting paradox. The PCT's highest point ( Forester Pass at 13,153') is almost twice as high at the AT's highest point (Clingman's Dome at 6,643'). Yet the total elevation gain/loss is MORE on the AT. (AT approximately 515,000 total vs PCT 315,000) ** note: there has been a bit of debate about these figures lately, as this year's Halfmile maps list the elevation gain/ loss at 489,418 ** Regardless, taking into account that the PCT is 500 miles longer, it translates to a more graded trail, fewer steep inclines (sometimes almost vertical--I'm talking to you, Maine) and fewer PUDs (pointless ups and downs). On the flip side, that also translates into lots of switchbacks, which means in a few months I may be complaining about PBFs (pointless back and forths).
Probably the greatest difference between the two is the amount of precipitation and availability of water sources. On the AT, water is plentiful. (as is rain) Except for a couple stretches in Pennsylvania, the next water source is never more than a few miles away.(The tunnel is green for a reason) On the other hand, the PCT has long stretches of waterless terrain, both in the desert and in higher elevations. Which means carrying more water (6-8 liters at times). On the flip side, the arid environment means no rain. After a particularly memorable 37 day stretch of daily rain (I'm talking to you, Virginia) I'm actually looking forward to hiking without squelching in my boots. I may change my mind about that once I'm in the desert heat, though.
Finally the number of people who attempt to thru hike is vastly different between the two trails. The AT, especially at the beginning, sometimes resembles a mobile party, with upwards of 50 people at the shelters and campsites. The PCT, because of its remoteness, sees a significantly lower number of hikers each year. However, this year there is a record high number of hikers registered to hike the PCT (possibly because of the WILD movie, possibly because word got out about the thru-life philosophy and record numbers of people are jumping on board) That's a good thing in my book. There more hiker-trash friends, the better.