Pasteurized process cheese, for example, is made from one or more cheeses, such as cheddar or colby, and may have cream or anhydrous milkfat added. The cheese is blended and heated with an emulsifiertypically a sodium or potassium phosphate, tartrate, or citrateand other optional ingredients such as water, salt, artificial color, and spices or other flavorings.
The cheese is then poured into molds to solidify and is later packaged. This processing produces a smooth, mild-tasting cheese that melts easily. For pasteurized process cheese, the final product can have a maximum moisture content of 43% and must have at least 47% milkfat. An interesting twist is that the product alternatively can be labeled as pasteurized process American cheese when made from cheddar, colby, cheese curd, granular cheese, or a combination of these; when other varieties of cheese are included, it must be called simply American cheese.
Here are some of the other definitions:
Pasteurized process cheese food is a variation of process cheese that may have dry milk, whey solids, or anhydrous milkfat added, which reduces the amount of cheese in the finished product. It must contain at least 51% of the cheese ingredient by weight, have a moisture content less than 44%, and have at least 23% milkfat.
Pasteurized process cheese spread is a variation on cheese food that may contain a sweetener and a stabilizing agent, such as the polysaccharide xanthan gum or the Irish moss colloid carrageenan, to prevent separation of the ingredients. The cheese must be spreadable at 70 F, contain 44 to 60% moisture, and have at least 20% milkfat.
Pasteurized process cheese product is process cheese that doesn't meet the moisture and/or milkfat standards.
Imitation cheese is made from vegetable oil; it is less expensive, but also has less flavor and doesn't melt well.
For the record, Velveeta is pasteurized process cheese spread and Velveeta Light is pasteurized process cheese product. Cheez Whiz is labeled as pasteurized process cheese sauce, although that type isn't noted in the Code of Federal Regulations. A Kraft spokeswoman confirms that the word "sauce" just seems to be an add-on.
No, it was just the first link that popped up in Google! 
