Chocolate is a common ingredient in many kind of sweets. It is made from the fermented, roasted, and ground seeds of the tropical cacao tree. The substance yielded is intensely bitter. In the U.S. this substance is usually sweetened and the sweetened product is referred to as chocolate.
The word "chocolate" comes from the Aztecs of Mexico, and is derived from the Nahuatl word xocolatl, which is a combination of the words, xocolli, meaning "bitter", and atl, which is "water".
When it was first introduced into European culture, chocolate referred to a bitter beverage, often with Chili pepper and/or Corn added. It became popular in Europe only after these ingredients were replaced with Vanilla and Sugar to make the beverage we now know as hot chocolate.
Chocolate Making 
To prevent the fatty skin that forms on hot chocolate, a defatted product known as cocoa was developed. The terms "hot cocoa" and "hot chocolate" are now often used interchangeably, but they denote a difference in the amount of cocoa butter in the beverage. Excess cocoa butter from this process is now used to make chocolate bars more durable and palatable, so that cocoa and chocolate are almost always made in tandem. As a general rule, elite cocoa makers (Hershey, Nestle) make good-but-not-excellent chocolate, and elite chocolate makers (Scharffen Berger, Ghiradelli) make good-but-not-excellent cocoa.
A further elaboration was the development of Dutch process cocoa, in which some of the acids in the chocolate liquor are neutralized to reduce its sour taste and allow more subtle flavors to come to the fore. A given amount of Dutch cocoa therefore tastes more chocolatey than the same amount of unprocessed cocoa. In the ingredients list of commercial foods, it is often listed as "cocoa processed with alkali".
Types of Chocolate 
There are many kind of chocolate, including:
Unsweetened or Baking Chocolate 
Unsweeted chocolate, or baking chocolate, is simply chocolate with nothing added. It has a very bitter flavor so is not often eaten plain, but it is used in baking.
Dark Chocolate 
Dark chocolate has no added milk, but does have sugar added, and is sometimes called plain chocolate. The U.S. government calls this Sweet Chocolate and requires 15% concentration of chocolate liquor. European rules specify 35% cocoa solids.
Researchers in Scotland and Italy say dark chocolate has much better anti-oxidant properties than milk chocolate - and much like drinking red wine in moderation, it may help protect against heart disease and cancer.
Bittersweet and Semi-Sweet Chocolate 
Bittersweet and semi-sweet chocolates are dark chocolates with high percentages of cocoa liquor (bittersweet having more than semi-sweet) and added sugar. They have a bitter flavor and are mostly used for cooking, but can be eaten plain. The two are interchangable in most recipes.
Milk Chocolate 
Milk chocolate is chocolate with milk and sugar added. The U.S. government requires 10% concentration chocolate liquor. European rules specify a minimum of 25% cocoa solids.
White Chocolate 
White chocolate is a confection based on cocoa butter and sugar, but not the cocoa solids.
Cocoa Powder 
Cocoa powder is an unsweetened chocolate with most of the cocoa butter removed and pulverized into a powder, making it more convenient for cooking since melting is not required.
Chocolate Chips 
Chocolate chips are small chunks of sweet chocolate, often sold in a round, flat-bottomed teardrop shape, usually 1 cm in diameter.
Storing Chocolate 
Chocolate is very sensitive to temperature and humidity. Ideal storage temperatures are between 15 and 17 degrees Celsius (59 to 63 degrees Fahrenheit), with a relative humidity of less than 50%.
Chocolate should be stored away from other foods as it can absorb different aromas. Ideally, chocolates are packed or wrapped, and placed in proper storage with the correct humidity and temperature. Additionally, chocolate should be protected from light by storing in a dark place or wrapping in paper.
Various types of "blooming" effects can occur if chocolate is stored or served improperly. If refrigerated or frozen without containment, chocolate can absorb enough moisture to cause a whitish discoloration, the result of fat or sugar crystals rising to the surface. Moving chocolate from one temperature extreme to another, such as from a refrigerator on a hot day can result in an oily texture. Although visually unappealing, these conditions are perfectly safe for consumption.