We humans have two hands, and typically one is dominant over the other. The dominant hand is stronger, more accurate, and more skilled at handling objects. If you throw an object to a person, they will typically catch it with their dominant hand.
Hand dominance is determined in the brain. The left side of the body is controlled by the right side of the brain, and vice versa. Some individual difference in brain structure determines if an individual person is left or right handed.
Now the weird part: 90% of humans are right handed, 10% are left handed. I would have expected the split to be 50/50, since there's no big advantage to either direction. Left handed people have had a "surprise" bonus in combat against right handed people, but this has not encouraged huge growth of left handed people.
Culture may have something to do with the disparity. The left direction is almost always associated with bad things for some reason. In pre-toilet-paper societies, the left hand is used for hygiene, making it undesirable to do anything else with it. Other societies have associated left handedness with dark magic, clumsiness (which has more to do with making the person operate against their natural handedness than they think), strangeness in general, and the ancient Hebrews associated the left hand with "the ability to shame society." I'm not clear if that reference means the ability to humiliate the entire tribe, or the tribe's ability to eject members.
Left handed people need their tools custom-made, because almost all tools are built with the assumption of a right handed user. Until recently, left handed people were "switched" at the time that they learned to write, punished when they used their left hand until they used their right hand exclusively. Many of these people wind up ambidextrous. Left handed people have been noted to have, in comparison with right handed people, more spacial skills, and are often excellent artists.
I am right handed. My grandfather was a "switched" lefty.
BREAKING NEWS: A reader of mine writes in to tell me that "switched" lefties often suffer from stuttering, anxiety disorders including obsessive compulsive disorder, and other ailments. This suggests to me that "switching" people is a really bad idea.