Techniques are all the non-magic skills that the characters can use. With the exception of Gun techniques, characters must have an empty space in their ability menu in order to learn a technique. Gun techs are awarded at the end of battle and are automatically placed under the gun skillset.
Any Human is able to learn any sword, gun, fighting or dodge technique. Obviously, some characters will be better at learning certain skill sets, which is usually found out by checking their starting equipment and learned skills.