1) Slutsky's theorem states that the total price effect on demand for a good is equal to the sum of the substitution effect and the income effect from a price change.
2) The substitution effect measures how demand changes due to consumers substituting between goods in response to the price change, holding their real income constant.
3) The income effect measures how demand changes in response to the change in purchasing power from the price change, holding utility constant.
4) According to Slutsky's theorem, the total change in demand from a price change can be decomposed into these two separate effects.