Fields are the most common Pokémon fighting areas. Wild Pokémon also attack unexpected trainers, although they an be clearly visible due to low grass.
Fields tend to have lots of grass, followed by a few trees, and sometimes vegetation growing in the bright sunlight
Rules and guidelines on the field is not very strict. Unless in the case of a gym battle, trainers get to choose what type of battle format they want, whether that be single, double, or triple battles.