Same units as are used to describe any other force arising from any other cause.
Some units of force are
newton
pound-force
dyne
ton
stone
ounce
The mass of the objects and the distance between the objects.
Gravitational attraction.
This phenomenon is controlled by gravitational pull. Large objects tend to have a large gravitational pull. Smaller objects are pulled in by large objects. The reason why planets are a steady distance from the sun is because they have a perfect amount of inward and outward pull.
The gravitational pull is always present: there is no "when".
Anything that has mass has a gravitational pull. I do not know the formula that determines an objects gravitational pull based on mass, but there definetly is one.
Mass, not density, and the closeness of objects, affects an object's gravitational pull. Density is not dependent on an object's size, but mass is. The more massive an object, and/or the closer an object is to another, the greater its gravitational pull.
Objects of greater mass have more gravitational pull.
Heavier objects have more gravitational pull on them
An object have greater gravitational pull closer from earth. As we get farther from earth, the gravitational pull becomes weaker. That is why objects sufficiently away from the earth do not fall on it.
mass and distance form an inverse relationship when related to gravity. The larger the mass(es) the greater the gravitational pull. The closer the distance, the greater the gravitational pull.
Any two objects with mass will have a gravitational force. The orbit of planets around stars depends on the gravitational pull of the star. The Earth exerts a gravitational pull on its moon but the moon also exerts a pull on the Earth.
Gravitational pull is only noticeable for large objects, stars, planets, moons. Smaller objects just don't have enough mass to make much difference.