The physics of general relativity is very simple:
MASS AND ENERGY CURVE SPACE AND TIME
and Einstein gave the equation which actually determines the relationship. The rest of general relativity is based on the mathematics of curved surfaces.
Tests of General Relativity
The most exciting test of general relativity is the PERIHELION OF MERCURY. Since the middle ages, it has been observed that the planet mercury travels along an ellipse (a squashed circle), and that the orbit shifts a little each year. The observed movement was a rotation of about 42 degrees per year. According to general relativity, the shift is 43 degrees per year, which convinced the world the general relativity was a correct theory.
Another test is the GRAVITATIONAL REDSHIFT. According to general relativity, light will change color as it gets closer to the surface of the Earth. When researchers carefully measured light at the top and bottom of a tall building, they found exactly the same result! Unfortunately it was later shown that the same effect occurs in quantum mechanics without using general relativity.
The third classical test of general relativity is called the DEFLECTION OF LIGHT. According to general relativity, the sun should make other stars appear to move (this is only an illusion, the stars don't actually move). Of course you can't see stars when the sun is in the sky, so this is hard to measure. But a group of researchers waited for a total solar eclipse, and then measured the effect and found that it agrees with general relativity. However, in 1960 it was shown that the exact same effect can be predicted using special relativity, and so it wasn't a good test of general relativity.
The fourth test, performed in 1964, was the Shapiro radar bounce experiment. A beam of light was sent to a distant planet and then it returned to Earth. The light was deflected as predicted by general relativity, and so far no other explanation has been given for these results.
The fifth test is still being planned. According to general
relativity, a spinning top which orbits Earth should wobble a very small
amount. The group working on the project have developed extremely precise
instruments to measure the tiny effect, but haven't tested it in orbit.
You may also want to read:
Cosmology Made Simple
Black Holes Made Simple