Chapter I
What it means
The kara is a circle, and a circle has no end: it stands for God, who is eternal. Worn on the wrist, usually the right one, it reminds Sikhs that they are bound to the Guru and to God, and that the hand which wears it should do no wrong.
Its plainness matters. It is made of steel or iron, not gold, to show strength and humility rather than wealth.
Chapter II
Where it comes from
The Five Ks were given to the Khalsa by Guru Gobind Singh when he founded the order in 1699, as outward signs of a disciplined life. The kara is the simplest of them and is often the first worn by children.
Chapter III
Where you see it today
The kara is worn by Sikhs throughout the world, by men, women and children, initiated or not. Many people of Punjabi heritage of other faiths wear one too, as a sign of identity.