Y.M.C.A. (Young Men's Christian Association) is a non-profit community service organization originally founded in London in 1844, as a men (and boys) only facility, the YMCA has changed to admit women (and girls). Activities that take place at the YMCA include swimming, basketball, working out, day care services, and more.
There used to be a equivalent group for women only called the YWCA.
In the U.S. the YMCA is a powerful religious activist group promoting "manly" (athletic) Christianity and the Social Gospel.