The space around a planet (generally a massive body) where the gravity of the planet has its influence on an object is gravitational field. Gravitational field depends on the mass (mass distribution) of the planet.
The concept of gravitational field comes form the notion that any object placed within the vicinity of given massive body (say planet) has some gravitational influence and exerts some amount of gravitational force toward its center. Thus gravitational field is used to explain gravitational property of the body. It is measured in (N/Kg). Gravitational field can also be expressed as the force exerted by the massive body on an object of 1 Kg mass.
Again like gravitational force, gravitational field depends on how massive the body is and how far the test object (object of 1 Kg mass) is located form the center of the massive body. Another important thing to note is that gravitational field is a vector quantity. It is a force field (Space around massive where test object of 1 Kg mass is forced to accelerate the massive body).