It was founded as the female version of Lord Robert Baden-Powell's Boy Scouts, on March 12, 1912. In September, 1908, a number of girls turned up to the first Scout Rally at 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 of guides are called Brownies. In Canada, the Guides are divided into multiple programmes depending on age: Sparks, Brownies, Guides, Pathfinders.