The goal of a game of chess is to capture the king chess piece. The king is the most important piece but is not the most powerful.
A king chess piece is the center of the game in all regards but is certainly not the most powerful. He can only move one space in any direction during a turn.
When the opponent has an opportunity to take the king on the next turn but the king also has an opportunity to escape then the king is "Checked". When the opponent is certain to take the king on the next turn and the king does not have an opportunity to escape then the king is "checkmated". This piece can also be used in a special move called castling in which the rook and the king switch places under specific circumstances.
The king is not only the center of game play but is also the center of chess set design in many cases. A tournament regulation chess set requires that the king chess piece meet certain size specification and that it be a specific proportion to the chessboard. Make sure that the king can also be clearly distinguishable from the queen if you want a tournament regulation set.
It's best for the king to have a height between 3 ¾ - 4 inches. This is not only a preferred height but also falls within tournament regulation. The base of the king should be 40-50% of the height for tournament regulation. All the other pieces in the set should be in proportion to the king.
The size of the chessboard squares are then often determined by the size of the king. The base of the king should be around 75% the size of a square. Square sizes of around 2 ¼ inches can work well for kings in the preferred size range.
The king can come in many different mediums and styles especially when considering them chess sets. In some sets it may not be immediately clear which piece is the king and which is the queen.