Handedness

Handedness is the preference for using either the left or the right side of the body for certain things. People are described as left-handed or right-handed when they prefer to write with their left or their right hand. They may prefer the use of certain hands for certain tasks.
Most people are right-handed. Many prefer to use their right eye, right foot and right ear if forced to make a choice between the two). The reasons for this are not fully understood. 
Ambidexterity is when a person has approximately equal skill with both hands and/or both sides of the body. True ambidexterity is very rare. In some sports work is done to even up the influence of the non-dominant side. Soccer is a good example.
Handedness and brain function.
Handedness is caused by the lateralisation of brain function. Lateralisation is not only found in humans: it is found in many kinds of animals.
Handedness seems to follow from the brain hemisphere division of labor. In most people the left side of the brain controls speaking, and the left cerebral hemisphere of the brain controls the right side of the body. In 90-92% of all humans, the left hemisphere is the language hemisphere. Therefore right-handedness predominates. This theory predicts that left-handed people have a reversed brain division of labor. 
In society.
Many societies have tried to force right-handedness onto left-handed children ("forced laterality"). The general experience is that such attempts are not entirely successful, and cause distress. Writing is a good example. A right-handed script moves towards the right, and the righthanded writer's hand reveals what has just been written, which is good feedback. When a left-handed person writes a right-handed script, their hand obscures (hides) what has just been written. That interferes with the natural visual feedback to the writer. So the way our alphabet is written, moving to the right, is better for right-handed writers. 
Always there are always more right-handed people in a population. But, all the same, the world has many important scripts which are written right to left. Some, like Arabic, Hebrew, Persian and Urdu are widespread and important. It is worth remembering that in the past most people could not write. Even in England in 1800 many men signed their marriage certificate with an X, because they were illiterate (could not write). Widespread literacy came in the 19th century. So, when writing was invented, it was a few thousand years ago, and only the priests and officials needed to use it. Almost all monuments of that time used pictures as well as words, so that the mass of people could understand them. Much that we take for granted, like spaces between words (!), was not there at the start of writing, and several oriental languages can be written top-to-bottom as well as horizontally.
On the other hand, with a keyboard, handedness does not matter. This is why the issue of handedness is not so important now as it once was.
However, there are many tools which are usually made for right-handed people, and it may be difficult and more expensive to buy left-handed versions.
Handedness and early mankind.
Physical anthropologists have pointed out that the human ability to throw weapons such as rocks and spears is quite notable. Chimpanzees have handedness, but their ability to throw is much less good. For protection, catching prey and for making tools, handedness is critical. Handedness may have been part of the mental make-up of Neanderthals, with asymmetric wear on their teeth as evidence. This would make sense, though it brings the question: "since its reason is not obvious, then why is it so widespread?". Even Darwin asked himself "Are monkeys right-handed??". In general, the answer appears to be yes.
Other animals.
Many other animals also have handedness. For example: elephants often have preferences for whether they swing their trunks to the left or the right. Honeybees have right antennas that are more sensitive to smells. Parrots can be left- or right-footed, and some don’t mind (they are ambidextrous). Animals as different as chickens and minnows like to look for food with one eye and look out for predators with the other. This seems to help them do two things at once.