Louis Pasteur performed the first scientific pasteurization in the mid 1800s. The key idea behind pasteurization is the reduction, but not necessarily the complete elimination, of bacteria in food products.
Long before Pasteur's time, people knew that food could be preserved by sealing it in airtight containers and heating it thoroughly; Nicholas Appert published the first book on how to preserve food this way in 1810. Much of the world still depends on food preserved by canning. However, some foods undergo undesirable changes of taste or texture when subjected to the levels of heat necessary for sterile canning. Dairy products, fruit juices, beer, and wine all react poorly to being boiled; these products are usually pasteurized instead.
Pasteurization is an exacting process of applying just enough heat for just enough time to kill most of the bacteria in a food, and particularly the pathogenic ones. The combination is product specific, but for example, either heating milk to 145°F for 30 minutes, or to 162°F for 16 seconds, will pasteurize it properly.
Pasteurization is widely credited with reducing rates of food borne illness in the US. Cases of tuberculosis, brucellosis, salmonella, and listeria have all dropped dramatically since the use of pasteurization became widespread.