Force fields are protective screens made of pure energy, generated by gravity-control tech, electric magnets, or a combination of both. They are used on medium and large spaceships, including Bulk cruisers, Asteroid ships, Basestars, Imperial Star Destroyers, and the largest of Corellian saucers.

They are also used to protect stationary targets on planets and moons, especially government buildings and military bases.  A direct hit from weapons of mass destruction will not usually destroy  targets protected by a force field powered by gravity-control tech, but often such direct hits will short out the force-field generator. 

A force field powered by gravity control will also protect a target from Kinetic-kill missiles, but enough direct hits from such missiles will eventually short out the force-field generator.

A force field powered by electric magnets is needed to protect a target from gunfire from blasters. However, enough hits from blasters will eventually short out the field generator.

A spaceship, building, or other target often will have a force-field generator with both gravity-control and electromagnetic tech.