K is the equilibrium constant. Therefore K is revealing the amount of products to reactants that there should be when the reaction is at equilibrium. Q is used to determine whether or not the reaction is at an equilibrium. At any given point, the reaction may or may not be at equilibrium.

Q is a quantity that changes as a reaction system approaches equilibrium. K is the numerical value of Q at the "end" of the reaction, when equilibrium is reached.

Q can be used to determine which direction a reaction will shift to reach equilibrium. If K > Q, a reaction will proceed forward, converting reactants into products. If K < Q, the reaction will proceed in the reverse direction, converting products into reactants. If Q = K then the system is already at equilibrium.

Q > K: When Q > K, there are more products than reactants resulting in the reaction shifting left as more products become reactants. Q = K: The system is at equilibrium resulting in no shift.