Gravitational force acting on an extended object, such that the portions of the object closer to the source of gravitation feel a stronger force than the portions farther away. Such a force, also known as a "tidal force," acts to deform or disrupt the object, and is responsible for many arising because the One of the four fundamental forces, gravitation is the force of mutual attraction that is exerted between massive bodies and between particles that have mass. Although gravitation is far weaker than the other three fundamental forces over short ranges, it is the dominant force on large scales because its range exerted on one body by a second body is not constant across its diameter. The side nearest to the second body experiences a greater force, while the opposite side experiences a lesser force.
Image source: http://www.pol.ac.uk/home/insight/images/bulge.gif.