Shields are referred to in loads of Sci-Fi films and television shows. Could they really exist one day? In truth they exist now, but not in the the form you'd expect. I am sure everyone reading this has tried to push simple bar magnets together with the same polarity, i.e. a North to North or a South to South. What is experienced is a force of repulsion - in essence this is a shield, and it has a very important application; protecting the people of Earth from the harmful radiation of the sun. The Earth is like a huge bar magnet with a North and a South pole. Harmful cosmic radiation from the sun is reflected around the magnetic field causing the beautiful auroras at the poles. It may be possible, in the future, to artificially create a magnetic field around a space craft to protect the crew from radiation, and generate some fantastic auroras around the ship in the process.
But could this be used as a shield as seen in science fiction? For example, could you create a magnetic field sufficiently strong to reflect a high speed bullet? In theory, the answer is yes. In practice, how you'd actually apply it is unknown. Basically, the strength of magnetic fields can be huge, for example astronomical measurement of neutron stars have recorded such high magnetic fields that all matter falling onto them is redirected to the poles of the star. Additional measurements have been undertaken which show that large sun's rotational speed can be slowed down by companion neutron stars or black holes.
How does this translate to Sci-Fi? Well, shields don't have to be just magnetics there are other forces as well but the same general pattern remains, i.e. perhaps on a large complex technologically advanced ship there could be sufficient power to generate a shielding system, but personal systems carried by people seem to be out of the question.