Basically, gravity is the attractive force between all objects of mass in the universe.  You are drawn by gravity to the Earth, but the Earth is also drawn by gravity (of equal force) to you.  It is a relationship of mutual attraction.

Gravity is more than just a handy thing to have around if you want to stick to the Earth.  Without it, or with it greatly reduced, we wouldn’t have an atmosphere (gravity holds it to the planet), or liquid water (it would boil away).  Escape velocity could be achieved on a trampoline, which is rather horrifying when you stop to think about it.

When we think of the study of gravity through history, modern man tends to think of Sir Isaac Newton sitting in a garden watching apples fall from the trees.  Cute, but… no.  In the first place, gravity was being written about scientifically since Aristotle in the 4th century BC, at least.  It was defined as an attractive force between objects of mass by Indian astronomer Brahmagupta in 628 AD.  What Sir Isaac Newton did (in 1687 AD) was define the concept of gravity to unify the existing concepts of celestial and terrestrial attraction.  Sir Isaac Newton didn’t know how gravity worked (we had to wait for Einstein to figure that one out), but he did put forward a simple, elegant, unified theory of gravity that stood unmatched for nearly 230 years.

Newton's Law of Universal Gravitation Image: Dennis Nilsson

As in all things scientific, the more you know about gravity; the more you realize you don’t know.  Science is still running up against the unexpected and inexplicable in gravity.  If you get to feeling frisky, you can read about some of these “who knows” moments in modern science:

The Fly-By Anomaly – In sling-shot maneuvers, spacecraft got more of a boost than they “should” have.

Accelerating Expansion – Here’s a biggie:  The expansion of space seems to be speeding-up instead of slowing-down.

The Pioneer Anomaly – The Pioneer spacecraft are slowing down.  They aren’t supposed to be.

The Dark Flow – Galaxy clusters are apparently being pulled toward something big.  Too big to have been formed since the Big Bang.  Something we haven’t seen yet.

We all understand the force of gravity at a fundamental level.  You have been aware of it since childhood, when you conducted scientific experiments to study the effect of gravity on your peas at the dinner table.  While our understanding of gravity has become somewhat more sophisticated, you’ll be happy to know that peas still react the same way when dropped from the height of a table.

Except, of course, fresh frozen peas bounce higher than canned peas.