King is the highest nobility achievable in the feudal system. A few nations allow women to hold the title of solitary king, thus called a queen, but most nations have primogeniture, meaning that the eldest surviving male will succeed the dying king, and the only time a woman may rule is when she is the sole surviving heir. A system where the eldest heir, regardless of gender, would succeed the king is called absolute primogeniture. Kings are the heads of an entire nation and rule the country through vassals, who in turn have vassals of their own. It varies from nation to nation just how powerful the king is and how much of the country that is under the direct control of the king. This area that is directly under the control of the king is called a demesne. While the king technically rules the entire country, the other parts of the nation is divided into sub-sections, usually called duchies, counties and baronies.
Kings are commonly appointed through some form of primogeniture, meaning that his heirs will succeed him. In the rare case that a king has no heir, his vassals will either choose one among their own to be the new king, or if they cannot agree, war over it and the winner takes the crown. Sometimes a new king will be selected through organised combat, such as a tournament.
A king is required to provide support to his vassals in case of war and can sometimes also be required to provide funds to his vassals, should they need it.
As king, one is the highest authority in the feudal society. This means that your vassals may use your land and take care of it, providing you with the profits, food and necessities from the used land, without having to directly rule it. A king is also highly respected provided it is a good king, or feared if it's a king who rules through force.
Grounds for Removal/Dismissal
A king is never removed from office until he is dead, or he appoints his heir as the successor. It is therefore common for kings to get assassinated by political enemies, foreign rulers or by displeased vassals.
High King, Queen (female), Kunning, Moh
Source of Authority
Commonly the king rules through the power of his vassals. A king who can ensure the loyalty of his vassals extend his power through said vassals.
Length of Term