A Ko is simply a pattern in which both side can eat each other "forever" if no rule is implemented to stop this as shown in as shown in the 4 diagrams below.

black places seed '1'white places seed '2'

From these diagrams , it can be seen that if white capture a black stone , black can capture white stone during his turn and white can capture another black stone ...... This goes on forever , so there came a rule which state that if one side captures a stone in Ko , the other side cannot recapture on the immediate next move , but can do so after his next move .

For example , in the diagram above, a Ko situation has occurred in the top right corner of the board . Black 1 capture the white stone first . Because white cannot recapture , he will have to play elsewhere . If black choose not to defend at the marked area , then white can capture the black stone . During black's turn , he cannot recapture the stone ,so he will have to play somewhere . Then it's white turn to decide whether to defend the Ko .

rule 6: suicide moves and eyes

rule 8: expert play