The first stop for crude oil at a refinery is the distillation tower. Crude oil is made of all different compounds of hydrogen and carbon, (hydrocarbons). Each hydrocarbon has a different boiling point. So when the oil is heated, the lighter compounds vaporize and rise to the top of the tower. As they condense, they are caught on horizontal trays on many levels thereby separating the oil. The separated oil factions run in pipes to the next process.
Many different processes are conducted at a refinery. These are different types of conversion which change the hydrocarbon molecule. After low grade oil goes through conversion processes to upgrade it and make it more valuable, the oil goes to the last major refinery process: blending.
Here different fractions of oil are blended together to make products like diesel fuels, jet fuels, and asphalts.