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.