The Girl Guides (known as Girl Scouts of the USA in the United States) is a scouting organisation for girls and young women. The World Association of Girl Guides and Girl Scouts is the international governing body of the girl scouting movement.
It was founded as the female version of Lord Robert Baden-Powell's Boy Scouts, in 1910 with the assistance of his sister Agnes Baden-Powell. In September, 1908, a number of girls had turned up to the first Scout Rally at the Crystal Palace, calling themselves Girl Scouts. Lord Baden-Powell set up the Girl Guides as a parallel movement for them, run by his sister Agnes Baden-Powell. While Agnes played a major role until her death, Lord Baden-Powell's wife, Lady Olave Baden-Powell, became Chief Guide of England in 1918, and World Chief Guide in 1930.
The Girl Guides were named after the famous corps of guides in India. Baden-Powell thought that to call them Scouts might alienate the boys, not to mention the girls' parents!
As girls are now allowed to join the Scouts in Britain, Guide numbers are declining there.
In Britain, the junior age range (7-10) of guides are called Brownies; younger girls (5-7) are called Rainbows, and older guides become Young Leaders or Rangers (the former concentrating more on becoming guide leaders, the latter more on advanced guide activities). In Canada, the Guides are divided into multiple programmes depending on age: Sparks, Brownies, Guides, Pathfinders.
Further details and history of the Girl Guides can be found in the article on Scouting.