The deflector shield is the first line of defense for any starship.  It has the ability to protect from natural and artificial occurrences.  These shields can protect from nuclear, magnetic, and many other radiated energies.  This system creates extremely focused and localized distortions in space.  Transmission grids on the exterior hull of the starship create a field that is, of course, shaped like the ship.  When something attempts to penetrate the shields, a large amount of field energy is sent to that point.  This creates a powerful spatial distortion.  How does this help the ship?  Well, let's look at it from a few points of view.
      If one would look at the weapon hitting the shield, it would appear to bounce off the shield.  On the other hand, if one were to be on board the weapon, it would seem that the location of the ship has changed.
     The field generators are located throughout the ship.  There are 5 on deck 10, 3 on deck 31, and 1 on each pylon.
     The biggest problem with the shields is the shield frequencies.  While the shields are up, it is difficult for sensors to penetrate it.  One of the ways to get around this is to allow for a "window" in the shields.  This problem also affects the transporters, so they cannot be operated at all.  The shield frequencies are not completely bad.  If the frequency of a radiation is known, the shields can be adjusted to specifically block that radiation.  Imagine a star going nova right next to your ship, I would be thankful for these shields.

 These are the locations of the shield generators.  Only 6 are shown, but there are 2 underneath the saucer, and 2 below saucer.