An oil tanker transports bulk liquids or gasses from offshore fields to shore which is then sent to the refinery.
Once refined, product tankers distribute gasoline, heavy fuel oil, and diesel to the tank farms. From there the cargo is transported by tanker truck, pipelines, rail tanker cars, or inland barges to the consumers.
The largest tanker vessel in the world is the Jahre Viking (formerly the Seawise Giant). It is 1,504 ft long and 226 ft wide. This is in a class of tankers called Ultra Large Crude Carriers which carry half a million tons of crude oil. Very Large Crude Carriers have a capacity of 250,000 - 300,000 tons. However, these vessels are too large to travel into some ports. For example, the Suez Canal can only accommodate tankers capable of carrying 140,000 tons of oil or less.