Being left-handed means that you use your left hand more often then your right hand. Left-handed people tend to be more dexterous and stronger with their left hands then with their right. If you use both hands equally, you are ambidextrous.
About 10% of the population are left-handed. We do not know what causes some people to be left-handed, although there appears to be some genetic influence. In the past being left-handed was discouraged, and during the Victorian era (and beyond) children who tried to write and perform other basic actions with their left hands were forced to switch over to their right hand. If you have ever tried to write with your non-dominant hand, you can see how this could adversely affect a child's education.
While we don't know what causes left-handedness, we do know that males are more likely to be left-handed than females, and that identical twins are more likely to be left-handed than non-identical twins (or singletons). People with Down's Syndrome, who are epileptic, or who are on the autism spectrum are also more likely to be left-handed.
A few interesting facts about lefthandedness:
Among the presidents of the United States of America there have been an unusually high number of lefthanders -- about one in three. In the 1992 elections, there was not even a righthanded candidate. The incumbent president Bush and his rivals Clinton and Ross Perot were lefthanders.
According to the 1990 U.S. Census, Left is the 62,465th most common surname in the United States. Lefthand is slightly more common,at 55,970th.
Many researchers claim that all polar bears are left-handed. Lemurs, galagos (bush babies) and parrots are usually lefthanded.
Left handedness isn't entirely genetic, as identical twins can have different handedness. They share the same handedness about 75% of the time, indicating that there is a genetic aspect to it.
Lefthanders are often called southpaws.