Force fields are shields that occupy a distinct physical space. Different mutants' force fields behave in different ways; for instance, by blocking different things. Most absorb or nullify physical force (thus the name), and some can also block energy or heat. Also, different forcefield-projecting mutants are often subject to different limitations: some are limited in range and can only protect themselves, while others can only create walls that are only useful in one direction, and yet others can only create small shields that cannot protect the entire body.
For the most part, force fields are used defensively, as such applications require very little effort beyond putting them between an attacker and his target. However, mutants who can create dynamic (i.e. moving) force fields sometimes use them as offensive weapons.
It is important to note the difference between force fields and other powers used to repulse attacks. For instance, many telekinetics can repel thrown objects, but this is not a force field. Rather than projecting a wall to protect themselves, these mutants mentally 'catch' the objects and throw them back.