This year marks the 100th anniversary of Einstein's "Year of Miracles," when he published landmark papers on three fundamental areas of physics. For that reason, 2005 has been declared the "World Year of Physics". So feel free to celebrate appropriately; just remember Einstein's admonition not to drink and derive.
So what were Einstein's three big breakthroughs that year? The most famous is the theory of Special Relativity, which describes how the laws of physics seem to change as you get close to the speed of light, including his famous E=mc^2 equation. This is different from his theory of General Relativity, which deals with the warping of space time, black holes, etc. That theory came 10 years later.
Einstein's second big idea was on the "photoelectric effect." In short, Einstein discovered that light acted like a particle, called the photon. Scientists already knew that light also acted like a wave. Einstein's thoughts on the photoelectric effect helped lead to the theory of quantum mechanics, or how subatomic particles actually work.
Einstein's third big concept involved "Brownian motion." If you watch a speck of dust in a drop of water, you'll notice the dust move around randomly, even though the water appears still. Einstein suggested that this was due to the water molecules jiggling around, randomly pushing the dust in different directions. Einstein was able to calculate the size of water molecules from this information, and it proved that substances were made of atoms long before electron microscopes allowed us to see individual atoms.
You can read more about Einstein's Miracle Year at this site.