Bourbon is a type of whiskey. It gets its name from Bourbon County in Kentucky where it originated. Bourbon tends to be amber-colored, and a little sweeter and heavier in texture than other whiskeys.
For a whiskey to qualify as bourbon, the law -- by international agreement -- stipulates that it must be made in the USA. It must be made from at least 51% and no more than 79% corn. There are no aging requirements for bourbon. However, most bourbon is aged for four years or more. If aged for a minimum of two years, and it does not have added coloring, flavoring, or other spirits, it may be (but is not required to) called straight bourbon. Bourbon that is labeled as straight that has been aged under four years must be labeled with the duration of its aging. This applies to Bourbon sold in the U.S.
The barrels for aging can be made of any kind of new oak, charred on the inside. Most distillers today use American White Oak because it is porous enough to help the bourbon age well, but not so porous that it will allow barrels to leak. It must be distilled at no more than 160 proof (80% alcohol by volume). Nothing can be added at bottling to enhance flavor or sweetness or alter color. The other grains used to make bourbon, though not stipulated by law, are malted barley and either rye or wheat.
Like Champgne isn't champagne unless it's made in the correct region of France, Bourbon is not actually "Bourbon" if it isn't made in the USA, even though other whiskeys may adhere to the same recipe and distillation guidelines.
edit Video explaining the differences
In this video Rebecca Dunphy differentiates between Scotch, Irish and Bourbon Whiskey by sniffing.
Prices vary by brand, age and type. Current prices for these are available on Amazon.com: