Crude oil is a complex mixture of many different hydrocarbons. It is not a single compound. Each hydrocarbon in the mixture is a compound of carbon and hydrogen only.
The molecules in crude oil have their carbon atoms joined in chains or in rings. The chains can be short or very long. You do not need to know the names or structures of specific ring molecules.
Crude oil is an important source of useful substances. Many of them are burned as fuels, and the rest are used as feedstock for the petrochemical industry, which makes other useful products from them.
Crude oil is a finite resource. It takes millions of years to form, and we use it much faster than it forms, so it will eventually run out.