Can you speak briefly about upstream and downstream in oil and gas field?

I leave the answer to experts and specialists in this field

Upstream: consists primarily of exploring for, developing and producing crude oil and natural gas; transporting crude oil by international oil export pipelines; transporting, storage and marketing of natural gas, and a gas-to-liquids plant. Downstream: Downstream operations consist primarily of refining crude oil into petroleum products; marketing of crude oil and refined products; transporting crude oil and refined products by pipeline, marine vessel, motor equipment and rail car, and manufacturing and marketing of commodity petrochemicals, plastics for industrial uses and fuel and lubricant additives.