Gravity is an attractive force exerted by every object with mass and felt by every object with mass.  Gravity is a field force, which means that it acts at a distance even if objects are not in contact.  Other field forces include magnetism and electrical force.  Newton explains that mass generates a gravitational field that affects the space around it.  This field is what interacts with other masses to create the gravitational force.

Gravitational force cannot be shielded or stopped.

Source: Galaxy NGC1, Jonathan Gibson, Form and Function

Gravity keeps planets in orbit, and holds stars together in galaxies. Everything in space moves at certain speed based upon strength of the gravitational field at that point. Very massive objects like planets, stars, and black holes can cause big changes in an objects motion.

Gravity from the moon causes the tides here on Earth. The moon pulls on the near side of the Earth stronger than it does on the far side. This difference in force has the effect of trying to stretch the Earth. The solid parts don't stretch very easily, but the liquid of the oceans do.

Source: Tidal_Forces, Windows to the Universe