explain the "THEORY OF RELATIVITY" in simple words

There are 2 parts to the Theory of Relativity: Special and
General. Special relativityonly deals with objects in uniform motion, while general
relativity deals with accelerated objects, and is a much harder subject for that reason.
According to special relativity, even for objects in uniform motion, strange and
unintuitive things can happen. If there was a passenger train inside which, through the
windows, you could see the passengers and their watches, and it whizzed by you, a
stationary observer, the following things will happen 1) the train will have shrunk
length-wise, and 2) the watches of the passengers will appear to be going more slowly,
and 3) the watches of the passengers in the back of the train will be AHEAD of the
watches of the passengers in the front of the train. When you get to general relativity,
more wierd things can happen. Time and space can be warped, and for this reason we have
gravity. That is the reason why Einstein's general theory of relativity is often
referred to as his theory of gravitation. General relativity explains why the universe
is expanding, and why there are bizarre objects like black holes, which are examples of
extremely warped spacetime.

It almost doesn't need to be said that
relativity says nothing can travel faster than light, and that E = mc², which is the
reason why we have atomic bombs.