HRH The Princess Margaret, Countess of Snowdon (Margaret Rose Windsor) (August 21, 1930 - February 9, 2002) was the younger sister of Queen Elizabeth II of the United Kingdom. Their parents were the Duke and Duchess of York, later King George VI and Queen Elizabeth.
Margaret fell in love with an RAF pilot and Battle of Britain hero, Peter Townsend, who was some years her senior. Townsend was divorced; had Margaret married him, she would have had to forfeit her place in the line of succession. Although she was only third in line to the throne, Margaret took advice from the Archbishop of Canterbury and senior politicians, and decided not to marry him, making a public announcement to this effect in October, 1955. Papers released in 2004 indicate she would have been allowed to keep her title, and her Civil List allowance. 
In 1960, she married photographer Anthony Armstrong-Jones, in a glittering ceremony, the first "modern" royal wedding thanks to the wider availability of television in the UK. In honor of his Welsh descent, her husband was created 1st Earl of Snowdon -- he was a son of Ronald Owen Lloyd Armstrong-Jones and his first wife, Anne Messel, later Countess of Rosse -- and, for the duration of her life, Margaret officially was known as HRH The Princess Margaret, Countess of Snowdon. They had two children, David Armstrong Jones, aka Viscount Linley, and Lady Sarah Armstrong-Jones, before getting divorced in 1978.
Margaret was well-known for being active in high-society circles, but was also involved in causes such as children's rights and supporting the arts. However, it was her indiscretions that always made headline news. She spent much of her spare time on the Caribbean island of Mustique, and had a relationship with a younger man, Roddy Llewellyn. (The revelation of this relationship led to her divorce; in reality her marriage had ended years earlier, with the Snowdons by the late 1970s barely being on speaking terms.) This led to her conduct being questioned by the media, and she was described in the House of Commons as "this wayward woman" by anti-monarchist MP, Willie Hamilton. Suggestions were made that she should be removed from the Civil List.
Margaret never remarried, although her ex-husband did (he married Lucy Davies, formerly wife of film director Michael Lindsay-Hogg, in 1978, and had further issue, including an out-of-wedlock son with an editor of Country Life). As royal divorces became commonplace, her past record appeared less scandalous, and she was once more respected for her charity work. Ill-health, partly due to heavy smoking, affected her later years, and her last public appearances were made in a wheelchair.
A 2003 ITV programme1 claimed that Margaret had had a two-year sexual relationship with American socialite Sharman Douglas (1929-1996), the daughter of Lewis W. Douglas, a United States Ambassador to the Court of St James (Britain). (Douglas, a mining and chemicals heiress who was briefly married to importer Anthony Hay from 1968 to 1977, reportedly confirmed the youthful liaison.) It further suggested that Margaret and her husband were potential targets for blackmail of Russian Spy Eugene Ivanov, whose affair with Christine Keeler (who was also having an affair with War Minister John Profumo) helped destabilise the government of Conservative prime minister Harold Macmillan. The programme also said that Princess Margaret developed a crush on Rolling Stones lead singer Mick Jagger and used cocaine. Though long rumoured, these allegations had not been published and broadcast in such detail before. The statement that she had a relationship with comedian and actor Peter Sellers was long known, having been confirmed by his family.
See also: British Royal Family