Queen

A queen is a woman who rules a country which is a monarchy. She may either get this position because of inheritance or because she has married a king.  

A queen regnant is a queen who is the monarch of a country. She usually comes to power after the death of the previous king or queen, who is her parent. A country that is ruled by a queen is called a kingdom – the same as a country ruled by a king. Elizabeth II was a Queen Regnant. 

A queen consort is a queen who is not a monarch, but is married to one, such as a King. Camilla is a Queen Consort. However, the husband of a queen is not always called a king. For example, King George VI had a wife, who was called Queen Elizabeth. He had no sons, so he was followed by his elder daughter, who was also called Elizabeth. She became Queen Elizabeth II. She was married to The Duke of Edinburgh until his death on April 9, 2021; but when she became Queen he did not become King. This is because the title "King Consort" does not exist.

Queen regnant
A queen regnant is head of state, and has all the powers of a sovereign.

A queen regnant does not keep the title of queen if she abdicates (resigns). No queen has ever abdicated in the history of the United Kingdom, but three abdications have occurred in the Kingdom of the Netherlands:
Queen Wilhelmina abdicated in 1948 and became Princess Wilhelmina.  Her daughter became Queen Juliana.
Queen Juliana abdicated and became Princess Juliana.
Queen Beatrix abdicated in 2013 and became Princess Beatrix.

 Queen consort 
A queen consort is the wife of a king.  She is not the head of state, and does not have any powers unless they are given by the King or another law.

The last King of Britain (George VI) had a wife (Elizabeth) who was called Queen Elizabeth. He had no sons, so he was followed by his elder daughter. She is also called Elizabeth and became Queen Elizabeth II. She was married to the late Duke of Edinburgh; but when she became Queen he did not become King.

 Queen dowager 

A queen dowager is the widow of a king.

A queen consort usually keeps the title of Queen after her husband's death, for example
Queen Alexandra, widow of King Edward VII of the United Kingdom
Queen Mary, widow of King George V of the United Kingdom and
Queen Elizabeth, widow of King George VI of the United Kingdom, she was given the title Queen Elizabeth the Queen Mother because her daughter is also called Elizabeth.