Henry and Eleanor
When William the Conqueror died in 1087 AD, he left the throne of England to his sons William II and then Henry, and Henry left it to his daughter Matilda. But when Henry died in 1135, his nephew Stephen of Blois grabbed the throne and wouldn't let Queen Matilda rule. Matilda fought Stephen for years, but she couldn't get into power.
Eleanor of Aquitaine
But when Stephen died, in 1154, Matilda's son Henry II and his wife Eleanor of Aquitaine took over England (and Normandy, and Aquitaine which belonged to Eleanor, so altogether it was about half of modern France as well). Henry was a strong king. He spent time organizing England, as well as fighting against the French, and against Eleanor, who led a rebellion against him.
The murder of Thomas a Becket
In Henry's arguments with the Church about whether the Church or the king was to be more powerful, some of Henry's men murdered St. Thomas a Becket, the archbishop of Canterbury, in his own church (in 1170).
Henry had four sons: Henry, Richard, Geoffrey and John. Henry, the oldest, died before his father, and so when Henry died Richard became king of England. Richard went on the Third Crusade, which was not very successful. While he was away, Richard left his youngest brother John in charge of England (because Walter had died). John had to collect a lot of tax money from the English people to pay for the Crusade. And then when Richard the Lionhearted was captured by the Holy Roman Emperor in Germany, John had to raise a huge amount of ransom money. So he became very unpopular.
In the Robin Hood stories, King John is the bad king who oppresses the poor and chases Robin Hood, and King Richard is the good king who comes back from his travels and pardons Robin Hood. Robin Hood probably never really existed, but the kings did.
When Richard died in 1199 AD, he had no children, and Geoffrey had been killed fighting in a tournament, so John became king after him.