The classic Stratocaster pickup configuration is three single-coil pickups (SSS), and there are many Strats that incorporate a humbucker at the bridge (HSS). However, there are also stock Strats with a pair of humbuckers, one at the bridge and one at the neck (HH) and even those with a single-coil pickup between the two humbuckers (HSH).
You'll find variations of these models at every price point, from Squier and Mexican-made Stratocasters all the way up to expensive American-made Stats.
The humbuckers give the Strat a thicker, warmer, heavier sound, and they are higher-output than standard single coils. A bridge-position humbucker means better crunch for genres like metal and hard rock. The neck position of a humbucker gives a Strat rounder, warmer sounds for jazz and rock soloing.
The interesting thing is, even with these pickup changes, to me a Strat still sounds like a Strat. In other words, an HH Stratocaster isn't going to sound anything like a Les Paul or even something like a Jackson or Ibanez. That Fender vibe always seems to come through.