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Oil consumption is caused by a few different things.
One of them is blowby. Expanding gases in the combustion chamber blow past the piston rings and entrain some of the oil lubricating the rings. This oily blowby is then evacuated from the crankcase by the PCV system and returned to the intake system; the oil is then burned in the combustion chamber.
Reverse gas flow is another one. During evacuation of the cylinder during the exhaust stroke, the cylinder pressure is actually lower than the pressure between the first and second piston rings. This causes a reverse flow of gas into the cylinder and it "sucks" some of the oil in liquid and/or vapor from the piston ring system into the cylinder.
Oil is also consumed as acceleration and deceleration of the piston throws oil from the top ring land into the combustion chamber. This is called "throw-off". Throw-off type oil consumption is worst at low loads.
In a similar way, aggressive driving can cause increased oil evaporation from the top ring land. Consumption due to evaporation also takes place at the cylinder liner. It's magnitude is highly dependent upon oil composition.
In older engines, there was some oil leakage from the valve guides but newer valve seal designs have virtually eliminated that as a source of oil consumption.
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