Hi,
The title "Dowager" is not always appreciated by the recipient of such a title. Also, several ladies were never known as "Dowagers" - - Queen Adelaide, Queen Alexandra, Queen Mary and Queen Elizabeth...The latter preferred 'Queen Mother'...
Queen Dowager is a title or status held by the widow of a deceased king. Queen Regnants aside, The "Queen" indicates someone who was a queen consort (i.e. wife of a king), while "Dowager" indicates a widow who holds the title from her deceased husband.
A Queen Mother is a particular type of Queen Dowager who is both a former queen consort AND the mother of the current monarch. Every Queen Mother is also a Queen Dowager, but the opposite is not true. For example, Queen Adelaide (consort of William IV) was a Queen Dowager but not a Queen Mother - since it was her neice rather than her child who succeeded her husband.
Finally, it is entirely possible for there to be one Queen Mother and one or more Queen Dowagers alive at any one time. This situation occurred in the UK from 6 February 1952 until the death of Queen Mary in 1953. For slightly over a year, there were three Queens:
Queen Elizabeth II, the reigning monarch
Queen Elizabeth The Queen Mother (who reportedly loathed being referred to as a "Dowager Queen").
Queen Mary, who had been Queen Mother between 1936 and 1952, but from 1936 always used the style "Her Majesty Queen Mary."
The Garter King of Arms proclamation of the styles and titles of Queen Elizabeth the Queen Mother at her funeral on 9 April 2002 illustrates her dual status as a queen dowager and a queen mother:
"Thus it hath pleased Almighty God to take out of this transitory life unto His Divine Mercy the late Most High, Most Mighty and Most Excellent Princess Elizabeth, Queen Dowager and Queen Mother, Lady of the Most Noble Order of the Garter, Lady of the Most Ancient and Most Noble Order of the Thistle, Lady of the Imperial Order of the Crown of India, Grand Master and Dame Grand Cross of the Royal Victorian Order upon whom had been conferred the Royal Victorian Chain, Dame Grand Cross of the Most Excellent Order of the British Empire, Dame Grand Cross of the Most Venerable Order of the Hospital of St John, Relict of His Majesty King George the Sixth and Mother of Her Most Excellent Majesty Elizabeth The Second by the Grace of God of the United Kingdom of Great Britain and Northern Ireland and of her other Realms and Territories Queen, Head of the Commonwealth, Defender of the Faith, Sovereign of the Most Noble Order of the Garter, whom may God preserve and bless with long life, health and honour and all worldly happiness."