What is the difference between active and passive earth pressure?

Active earth pressure develops when a retaining wall moves away from soil, allowing soil to expand laterally - this is the minimum horizontal pressure. Passive earth pressure develops when wall moves into soil, compressing it - this is the maximum horizontal pressure. At-rest pressure occurs with no wall movement. Active pressure Pa = 0.5*Ka*gamma*H^2, Passive Pp = 0.5*Kp*gamma*H^2, where Ka < Ko < Kp. Active condition requires less wall movement than passive.