There are a few different definitions of what Crude Oil really is. In simple terms, Crude Oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface.
Another word for Crude oil is Petroleum. This coming from the Greek "petra" which means rock and the Latin "oleum" which means oil. In some ways we could then call it rock oil. (based on the geology and mineralogy)
The classical, but incorrect, theory about crude oil treats it as a fossil fuel. However, hydrocarbons, like oil and natural gas, have been discovered on other planets and also proven to be generated in an abiotic environment, hence the name of the opposing theory about the origins of oil.
You can read more on the subject of Crude oil on Wikipedia. For those who are a little more interested than the average Joe. (there is a lot of information about equations for thermal properties and so on)