In terms of elements, it looks like this:
Built with LabMike's Interactive Periodic Table
Oil is primarily just two elements
Oil is primarily made of just hydrogen and carbon, and by putting these two elements together we call them hydrocarbons. Another common name for oil is petroleum, which is latin for "rock oil." They may be in the form of very long chains, like in parrafin wax or petroleum jelly, or they might be in the form of short chains, such as octane in gasoline (which is where the phrase "octane rating" comes from). They might also be in aromatic forms, like benzene, toluene, and xylenes (some of these can be found at hardware stores). We get all of these materials out of oil by distilling the oil into its separate parts, and by cracking the long-chain parts in oil to make them into smaller parts (i.e. turning asphalt and waxes into gasoline). When the oil is made up of smaller, free-flowing hydrocarbons, it's called light oil, and if it's made up of larger molecules that don't flow as easy, it's called heavy oil.
Oil also contains nitrogen, oxygen, and sulfur, which all lower the quality of oil and make it harder to work with. Sulfur is a pollutant and usually the most important one to remove, and oil with less than 0.5% sulfur is called "sweet" wheras oil with more than that is called "sour." Since sour oil is more expensive to clean than sweet, it is also less valuable.