Processed cheese is made from natural cheese that has been ground up, blended with emulsifying salts (like sodium citrate), heated, and then formed into slices, blocks or spreads. American cheese slices are the classic example.
Why it behaves differently: the emulsifying salts keep the fat and protein from separating when it melts. That's why processed cheese melts into a smooth, silky layer on a burger and never turns greasy, and why it keeps for so long. The trade-off is a milder, more uniform flavor, since it lacks the character that comes from aging.
Read the label name. "Pasteurized process cheese" is mostly cheese. "Cheese food" and "cheese product" usually contain less actual cheese and more added ingredients, such as milk or whey, so they're a step further from the real thing.
Want the smooth melt without the processing? See Why does melted cheese turn greasy or stringy?
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