The Fender Stratocaster has three single-coil pickups (bridge, middle and neck), selectable by a five position lever switch. The lever switch originally had three positions, but guitarists used to balance the switch between positions to get an out of phase sound, and these extra positions became standard in 1977. Positions 1, 3 and 5 activate only one pickup (bridge, middle or neck respectively), while positions 2 and 4 activate a combination of two pickups (bridge and middle, or middle and neck, respectively).  

Pickup position, number of coil winds, wire types, magnets and other factors shape the tone. Pickups in the neck position usually give louder, mellower and warmer sound, while bridge pickups have lower output and produce a brighter, sharper and more harmonic-rich tone. The reason the neck pickup has the most output is that the string's vibration has a higher amplitude at the neck position. To balance the output of the three pickups, some manufacturers use different coils in each one

The original Stratocaster pickups had staggered pole pieces from 1954 to late 1974 and then flush pole pieces from 1974 to 1981. The staggering of the pole pieces was supposed to compensate for the differences in the output of the individual strings. These differences arise due to the camber of the fretboard and the fact that some of the strings have naturally higher output, the plain or unwound G string being the most significant and this calls for these magnets to be further compensated, resulting in an apparent odd looking stagger.

Sources: The Fender Stratocaster, by A. R. Duchossoir. Published by Hal Leonard Corporation