What is an equilibrium?
An equilibrium system is where the rate of the forwards reaction equals the rate of the reverse. This can only occur in a closed system with a reversible reaction. An equilibrium system can be disturbed by changing concentration, pressure or adding a catalyst; this disables equilibrium. Also at equilibrium the amounts of products and reactants stay the same. Another name for this is dynamic equilibrium "the rate of the forwards reaction is equal to the reverse".

Homogenous vs Heterogenous

A homogenous reaction is where all the reactants and products (species) are in the same state. Whereas if the mixture is heterogeneous, only gases and aqueous solutions go into the Kc expression; liquids and solids are left out. In a heterogenous reaction, the species are in different states.

Le Chatelier's Principle

If there is a change in concentration, pressure or temperature the equilibrium will move to help counteract the change.

If equilibrium shifts to the right
K increases.

If equilibrium shifts to the left
K decreases.

Temperature

Temperature is a condition meaning it affects the value of Kc. If you raise the temperature, equilibrium favours the endothermic direction. Therefore, if the reaction is exothermic in the forwards reaction, equilibrium will shift left and the value of Kc will decrease. Where as, if the reaction is endothermic in the forwards direction, equilibrium will shift to the right and K will increase.

Catalyst

A catalyst has no effect on the position of the equilibrium or the value of Kc. It doesn't increase yield but it means equilibrium will be reached quicker.

Concentration

Kc will stay the same. However, in order for the value of the expression to equal the value of Kc, the other concentrations will need to change.

Pressure

Increasing the pressure shifts the equilibrium to the side with the fewest moles of gas. K stays the same no matter what you do to the pressure.

Equilibrium Part II - Summary