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The problem with crude oil is that it contains hundreds of different types of hydrocarbons all mixed together. You have to separate the different types of hydrocarbons to have anything useful. Fortunately there is an easy way to separate things, and this is what oil refining is all about.

Different hydrocarbon chain lengths all have progressively higher boiling points, so they can all be separated by distillation. This is what happens in an oil refinery - in one part of the process, crude oil is heated and the different chains are pulled out by their vaporization temperatures. Each different chain length has a different property that makes it useful in a different way.
To understand the diversity contained in crude oil, and to understand why refining crude oil is so important in our society, look through the following list of products that come from crude oil:

Petroleum gas - used for heating, cooking, making plastics
small alkanes (1 to 4 carbon atoms)
commonly known by the names methane, ethane, propane, butane
boiling range = less than 104 degrees Fahrenheit / 40 degrees Celsius
often liquified under pressure to create LPG (liquified petroleum gas)
Naphtha or Ligroin - intermediate that will be further processed to make gasoline
mix of 5 to 9 carbon atom alkanes
boiling range = 140 to 212 degrees Fahrenheit / 60 to 100 degrees Celsius
Gasoline - motor fuel
liquid
mix of alkanes and cycloalkanes (5 to 12 carbon atoms)
boiling range = 104 to 401 degrees Fahrenheit / 40 to 205 degrees Celsius
Kerosene - fuel for jet engines and tractors; starting material for making other products
liquid
mix of alkanes (10 to 18 carbons) and aromatics
boiling range = 350 to 617 degrees Fahrenheit / 175 to 325 degrees Celsius
Gas oil or Diesel distillate - used for diesel fuel and heating oil; starting material for making other products
liquid
alkanes containing 12 or more carbon atoms
boiling range = 482 to 662 degrees Fahrenheit / 250 to 350 degrees Celsius
Lubricating oil - used for motor oil, grease, other lubricants
liquid
long chain (20 to 50 carbon atoms) alkanes, cycloalkanes, aromatics
boiling range = 572 to 700 degrees Fahrenheit / 300 to 370 degrees Celsius
Heavy gas or Fuel oil - used for industrial fuel; starting material for making other products
liquid
long chain (20 to 70 carbon atoms) alkanes, cycloalkanes, aromatics
boiling range = 700 to 1112 degrees Fahrenheit / 370 to 600 degrees Celsius
Residuals - coke, asphalt, tar, waxes; starting material for making other products
solid
multiple-ringed compounds with 70 or more carbon atoms
boiling range = greater than 1112 degrees Fahrenheit / 600 degrees Celsius
You may have noticed that all of these products have different sizes and boiling ranges. Chemists take advantage of these properties when refining oil. Look at the next section to find out the details of this fascinating process.

Crude oil is the term for "unprocessed" oil, the stuff that comes out of the ground. It is also known as petroleum. Crude oil is a fossil fuel, meaning that it was made natural­ly from decaying plants and animals living in ancient seas millions of years ago -- most places you can find crude oil were once sea beds. Crude oils vary in color, from clear to tar-black, and in viscosity, from water to almost solid.

Crude oils are such a useful starting point for so many different substances because they contain hydrocarbons. Hydrocarbons are molecules that contain hydrogen and carbon and come in various lengths and structures, from straight chains to branching chains to rings.

There are two things that make hydrocarbons exciting to chemists:

Hydrocarbons contain a lot of energy. Many of the things derived from crude oil like gasoline, diesel fuel, paraffin wax and so on take advantage of this energy.
Hydrocarbons can take on many different forms. The smallest hydrocarbon is methane (CH4), which is a gas that is a lighter than air. Longer chains with 5 or more carbons are liquids. Very long chains are solids like wax or tar. By chemically cross-linking hydrocarbon chains you can get everything from synthetic rubber to nylon to the plastic in tupperware. Hydrocarbon chains are very versatile!
The major classes of hydrocarbons in crude oils include:

Paraffins
general formula: CnH2n+2 (n is a whole number, usually from 1 to 20)
straight- or branched-chain molecules
can be gasses or liquids at room temperature depending upon the molecule
examples: methane, ethane, propane, butane, isobutane, pentane, hexane
Aromatics
general formula: C6H5 - Y (Y is a longer, straight molecule that connects to the benzene ring)
ringed structures with one or more rings
rings contain six carbon atoms, with alternating double and single bonds between the carbons
typically liquids
examples: benzene, napthalene
Napthenes or Cycloalkanes
general formula: CnH2n (n is a whole number usually from 1 to 20)
ringed structures with one or more rings
rings contain only single bonds between the carbon atoms
typically liquids at room temperature
examples: cyclohexane, methyl cyclopentane
Other hydrocarbons
Alkenes
general formula: CnH2n (n is a whole number, usually from 1 to 20)
linear or branched chain molecules containing one carbon-carbon double-bond
can be liquid or gas
examples: ethylene, butene, isobutene
Dienes and Alkynes
general formula: CnH2n-2 (n is a whole number, usually from 1 to 20)
linear or branched chain molecules containing two carbon-carbon double-bonds
can be liquid or gas
examples: acetylene, butadienes
To see examples of the structures of these types of hydrocarbons, see the OSHA Technical Manual and this page on the Refining of Petroleum.

Now that we know what's in crude oil, let's see what we can make from it.