The Silverado Trail sounds like something you might hike (or at least take a mule ride along), and indeed this was probably originally the case in the late 1800's when silver miners came here to seek their fortune. Now it is one of the two major roads that run through Napa Valley. The main road, with which most people are familiar, is St. Helena Highway/Hwy 29, which runs north-south along the west side of the Valley. Highway 29 is home to some of the most-known names in the Valley: Mondavi, BV, Grgich, Franciscan, Schramsberg, Behringer, Sutter Home, Rubicon (Francis Ford Coppolla's winery)
Silverado Trail runs more or less parallel to 29, but on the opposite (east) side of the valley. It gets less attention probably because the wineries are not nearly as dense and plentiful on that side... however, what it lacks in quantity (and trust me, there are still plenty) it makes up for in quality. Silverado Trail has some big, well-known names too, the most famous probably being Stag's Leap (whose cabernet sauvignon won for best red in the "Judgment of Paris" wine tasting in 1976, featured in the film "Bottle Shock") and Mumm (sparkling wine). This side of the valley is where some of the best cabs are to be found, especially in the Rutherford and Stag's Leap AVAs. Excellent cabernets we have had from the Silverado side include Caymus, James Cole, and Chimney Rock (all of these wines were about $75/bottle)
Personally, I use the Silverado Trail whenever I drive to work, mostly because it is close to where I live and has less traffic than 29 -- this is especially true when dealing with St. Helena. However, I also drive Silverado in my convertible when people come and visit during the summer, because I feel like this part of the valley has the best natural scenery that can't be beat.