Field in this context refers to a region of the space to which corresponds a value.
There is a gravitational field around the earth, because a mass m placed at any point around the earth will be atracted (gravitational force) by it.
There is an electric field in a point when a charge placed there feels an electric force.
The gravitational field is proportional to the value of the mass of the object that produces it.
The electric field is proportional to the magnitude of the charge of the particle that produces it.
The gravitational field is always attractive. The electric field may be attractive or repulsive.
Both fields are proportional to the inverse of the squared distance.
The magnetic field is created when a charge is in movement,i.e a charge in movement will create a magnetid fiedl around it that will act and create a magnetic force over other charge also in movement.
The magnetic field is proportional to the product of the charge times its velocity and inversely proportional to the squared distance. The force generated my be attractive or repulsive.
