Both. For example the UK doesn't have a king now, even though our Queen is married. But her father was King and her mother Queen. It depends whose family the crown 'the power' is in. If the male is inherits the crown he will become King and his wife is queen. But if the female inherits the crown she might not make her husband king, because the king is more powerful than the queen (usually).

It means both! If the king marries a woman, she becomes the queen. However, if the king dies (or if there is only a female heir to the throne), it refers to the woman who rules the country, like Queen Elizabeth in England today.

Queen means both a ruler of a country, or the wife of a king. There is no equivalent for men, which is why most consorts of queen's remain with the title of prince.