I'll start with ordinary, everyday stuff first and then stuff gets weird.
Here on earth, gravity pulls stuff down. In fact, that's what we
mean by "down" -- it's the direction in which stuff falls when you drop
it. The ordinary ("Newtonian") view of gravity is that it is a force
which attracts objects to each other. Forces can bend objects. If you
hold a long, bendable object on one end, gravity pulls down on the
whole thing, and the object will bend downwards. A diving board will
bend under its own weight. Gelatin dessert bends under its weight.
People who design buildings and bridges and such stuff have to worry
about the weight of all the material pushing down on supports, bending
things. I once heard on the radio show "Car Talk" that even the hoods
of some cars can sag downwards if they are not properly braced. Wires
between telephone poles and electricity distribution towers sag under
their own weight.
Gravity bends the paths that objects take when they move. A
baseball, if thrown in space far away from earth's (and the sun's and
the galaxy's and...) gravitational field, will travel in a straight
line at a constant speed. Throw it on the earth, and this straight line
gets bent so that the ball falls down after going up. The actual path
is a parabola (air resistance changes that a bit, but that's the idea).
A parabola is a bent straight path.
Einstein predicted that gravity bends the paths of light rays too,
and this effect was observed shortly after its prediction. Since then
it has been verified lots of times. Search our site for the words
"gravitational lensing" and you will find more about this.
Stuff gets weirder still. Einstein effectively said that the paths
light takes are "straight", and it's really space and time that are
"bent". There are lots of consequences of this idea, such as the
prediction of black holes, gravitational radiation, and a change in the
frequency of light as it goes into or out of a gravitational field. All
of these ideas have strong experimental support.
(republished on 07/12/06)