You may have heard people describe themselves as strictly "right-brained" or "left-brained," with the left-brainers bragging about their math skills and the right-brainers touting their creativity. That's because the brain is divided down the middle into two hemispheres, with each half performing a fairly distinct set of operations.
Much of what is known about brain function is owed to Roger Sperry, whose experiments examined the way the human brain's hemispheres operate both independently and in concert with each other. The two hemispheres communicate information, such as sensory observations, to each other through the thick corpus callosum that connects them.
The brain's right hemisphere controls the muscles on the left side of the body, while the left hemisphere controls the muscles on the right side of the human body. When you wink your right eye, that's the left side of your brain at work. Because of this criss-cross wiring, damage to one side of the brain affects the opposite side of the body.