Most people have learned that gravity is what keeps objects on the Earth. Many have not learned that any object with mass has gravity; the more mass something has, the more gravitational pull it creates. Therefore, very massive objects like the Sun and Jupiter have significantly more gravity than Earth. Gravity keeps the Moon going around the Earth, the Earth going around the Sun, and the Sun going around the center of the Milky Way. Gravity is the weakest of the four fundamental forces of nature (electromagnetic, weak nuclear, and strong nuclear are the other three), yet it is the force that governs motion in the universe.
Tides - The Moon's gravitational pull tugs on Earth - especially the portion that is nearest to it as it travels in orbit around Earth. Earth's crust rises slightly (several centimeters) twice a day, due to this force. Ponds and lakes - such as the Great Lakes - experience small tides, as well. Earth's oceans, however, are free to lift many feet in response this tug. As the Moon orbits the Earth, it drags along behind it a "bulge" in the oceans.
If you've ever tried to push a stalled car in neutral from a dead stop, you've experienced the same force that counteracts the Moon's gravitational pull: inertia. On the side of Earth opposite the Moon, the universal tendency for objects to resist a change in its movement wins out over the gravitational pull. The ocean here can keep going in its original direction. Near the Moon, the oceans are pulled upward and along. Opposite the Moon, the oceans are "left behind."
The Moon's contribution to Earth's tides is significant because it is so close. The Sun, of course, also exerts a gravitational pull on Earth - that's what keeps Earth in a steady orbit. Earth's oceans are pulled toward the Sun, but the Sun's gravitational pull contributes only about a third of the tides' height.
Early in Earth's history, the Moon was even closer to Earth. Billions of years ago, the Moon was 10 times closer and tides were 1000 times higher. Scientists believe that these extreme tides occurred once every three hours because the Earth was spinning more rapidly. The tides eroded the coastal areas, adding minerals to the oceans. These minerals may have been essential for life to evolve as quickly as it did.
For more information about tides, go to the National Oceanic and Atmosphere Admistration's educational tides tutorial.
In Einstein's General Theory of Relativity, space is transformed from the Newtonian idea of a vast emptiness that has no effect on the motion of matter, to an invisible web of spacetime that grips matter and directs its course. Gravity is no longer simply a force that attracts matter; it is also the curve and warp of spacetime itself.
Measuring Gravitational Fields of Planetary Bodies
Classical Results and Techniques
Measuring Gravity on Earth
Measuring the Gravitational Field on the Moon
YSS Education and Public Outreach Materials
These sites provide basic detailed information about gravity