Consumption is a general term for motor oil loss – getting used or going away one way or another. It includes leaks, crankcase blow-by, and combustion usage. Some cars have none of these, or at least only in negligible amounts. Some can be significant, and others can be extensive. Of those, some might be easy and inexpensive to remediate – such as replacing a valve cover gasket or oil pan gasket or oil filler cap. Others can be cost-prohibitive such as a rear crankshaft seal or front main seal or camshaft seal.
Combustion consumption is when motor oil is consumed during combustion and is burned off with the fuel. Often this is accompanied by blue exhaust smoke and fouled spark plugs, but not always. It can be caused by worn piston rings or faulty valve seals.
Head gaskets serve to preserve continuity in the combustion, engine coolant, and oil chambers – the first two of which have very high pressures. A faulty head gasket can lead to the loss of pressures, fluids, and performance. They never get better and always get worse. Repair requires removal of the affected engine head at a minimum. It can be quite expensive.
When you go to check out a car to buy, if it is low on oil, consider it might be a sign of neglect or distress.
Be sure to check out the other oil-related posts.