Pasteurized liquids have undergone pasteurization, a process which involves heating to a specific temperature for a specific length of time, then rapidly cooling. This destroys some of the bacteria that are present, partially sterilizing the liquid in question and giving it a longer shelf life. Pasteurization, in combination with refrigeration, is a method of short-term food preservation.
The process was discovered by - and named for - Louis Pasteur. In the 1860s, at Louis Napoleon Bonaparte's request, he was researching the cause of wine spoilage and proved that if wine were heated to 135°F (57°C) the microorganisms that caused it to go bad would be destroyed. Today beer and some wines and fruit juices are pasteurized, but the main target is milk, which once carried bacteria that transmitted serious diseases such as typhoid, tuberculosis, polio, and dysentery. Milk is pasteurized by either heating it to 145°F (63°C) for 30 minutes or - the "flash" method - heating it to 160°F (71°C) for just 15 seconds (I have no idea how they do that); in either case it is then rapidly cooled to below 50°F (10°C). This does not kill all the bacteria: the lactic acid bacteria which cause milk to turn sour are not destroyed, which is why milk must still be refrigerated.
A newer process is ultrapasteurization, often used on milk, whipping cream, or liquid eggs; it involves heating the liquid in question to a very high temperature - 280°F (139°C) for two seconds, followed by rapid cooling. Ultrapasteurized products are usually sold in brick-style cartons and have a longer shelf life than other dairy products when refrigerated. Ultrahigh temperature or UHT products are heated to over 300°F (150°C) for at least 2 seconds; they are packaged in special multi-layer containers that allows them to be stored at room temperature, though they must be refrigerated once opened. The ultra-high temperatures compromise the nutritional value of products, destroying beneficial vitamins.
Critics charge that pasteurization destroys beneficial as well as harmful organisms and makes the product taste different. It renders the product unsuitable for some purposes; the home cook cannot make real clotted cream or crème fraîche with pasteurized heavy cream, for example. Sadly, in some countries - like my own, Canada - it is illegal to buy or sell unpasteurized dairy products. The only way you can get raw milk is to know a dairy farmer willing to risk prosecution by selling his fresh unpasteurized milk to you.