Global fields are found in the System panel and they include directional gravity, wind, electricity and magnetism. Electricity works like the Push space warp by applying positive or negative force to particles along a vector. Magnetism is a vortex-like effect causing particles to swirl around. Gravity is the only global field that affects particles absolutely. The others influence can be controlled by parameters within the particle rollout.
Wind only affects particles with positive or negative drag.
Along with the global forces, there are localized versions of Electric and Magnetic. These only affect particles relative to their areas of influence. I ve done some complex swirling particle motions with groups of localized magnetic fields. The custom fields work a couple of different ways. They can push particles along a vector or they can be programmed with an expression to exert a custom force on particles.