For decades the vehicle identification number on cars was different for every manufacturer. Some of them included factory location, model information, engine and transmission options, body style, etc, as well as dates and a serial number. Others were just a running number of what came down the assembly line or stamped as it rolled out of the factory.
In regulating safety and smog standards, the National Highway Traffic Safety Administration (NHTSA) standardized the VIN for all cars to be sold in the United States. Now, for all cars sold in the 1981 and later model years, you can look at the 17 digit VIN and discover many things about the car, like exactly when and where it rolled out of the factory. Most car, truck, and motorcycle companies worldwide use a NHTSA standard 17 digit VIN following the dictates of the American Society of Automotive Engineers, even if they don't sell cars here.
The first three positions are the World Manufacturer Identifier (WMI), with each country and company assigned a code. Additionally, the third digit is sometimes used internally to separate different divisions, like Chevrolet cars (1G1) from Pontiac cars (1G2), or Chevrolet trucks (1GC) from GMC trucks (1GT). You can use this number to see if your Toyota came from the NUMMI plant in California (1NX), the US truck plant in Texas (5T), one of the other Toyota US plants (4T), or all the way from Japan (JT).
To alleviate confusion, the letters I, O, and Q are never used in a VIN, but the numbers 1 and 0 can be. The letters A-H are assigned as African WMI. The letters J-R are all used in Asia. All cars manufactured in Europe start with S-Z. North America gets the numbers 1-5. The small island nations of Oceania (including Australia and New Zealand) use numbers 6 and 7. While all of South America uses 8 and 9.
The next five positions are the Vehicle Descriptor Section (VDS), which is where you will find information about what vehicle platform was used, engine installed, body style fitted, etc. Once you learn a thing or two about particular brands and models, this section can make sorting online searches quicker and easier. With eBay and Craigslist sellers listing the VIN on more and more ads, it only takes a few seconds to tell a V8 powered car from a four cylinder, and a crew cab from an extended cab, from the VIN.
The ninth position is also a check digit, which allows you to verify the authenticity of the VIN, or at least verify that it is legitimate. The system involves a lot of simple math, but it is not a simple process. You can find VIN check digit programs online, or read how to do it yourself here.
The next eight number or letters are the Vehicle Identifier Section (VIS), used to identify your specific vehicle, including model year, plant, options, production number, etc., and they vary by manufacturer. The last six digits are your car’s unique production number.
The tenth digit is always used to identify the model year, with the letter A designating 1980 and Y being 2000 (again skipping I, O, and Q, as well as U and Z), then 2001 through 2009 using the last number of the year. With 2010 it began all over again with A and will reuse A again in 2040. It isn’t likely that you will confuse a 1980 with a 2000 model year vehicle, but to keep registration records straight (so two cars with the same VIN don’t exist in DMV records) all of the earlier cars use a number in the 7th position, while after 2010 the 7th position must be a letter. Buses, heavy trucks, trailers, and motorcycles can actually repeat VIN numbers now, but it is not very likely to happen.
The 11th digit is used to identify the actual factory your car was made, in the event that the manufacturer has several assembly lines building them. This is very important to know in case of recalls for manufacturing defects. Sometimes a batch of parts used in one factory can be traced to causing all the problems, but the same vehicle made at a different factory on the same day is fine.
You can find plenty of information online about your car via a VIN decoder site.
The easiest place to look for the VIN is somewhere on the dashboard, typically visible through the windshield from outside. This is so police writing parking tickets can identify the car, whether you have license plates on it or not.
The next easiest place you can typically find it is on a sticker on the door jam. Most cars have it on the door frame on the driver side, along with proper tire inflation information and other data. Occasionally you will find it on the passenger side door though.
Under the hood, you can also typically find it in one of several places. Sometimes there is a sticker with emissions information near the radiator that also includes the VIN. Other times it is up on the firewall near the windshield. Finally, on some cars, there is a VIN stamped into the motor, or on a plate attached to it.