How does heating and packaging influence the lifespan of milk?

Pasteurization: Milk is heated to 145° F or 161° F, to destroy the pathogenic bacteria that cause disease, illness, and some food-spoilage bacteria. This process gives a regular carton of unopened milk a shelf life of up to 21 days.
 
Ultra-pasteurization: This higher  pasteurization temperature (280° F or higher) kills additional food-spoilage bacteria and extends shelf life to 60-90 days. Milk is then placed in sanitized containers and refrigerated. Dairy processors commonly (though not always) ultra-pasteurize organic, flavored, lactose-free, and single-serve milk to ensure freshness in products with lower retail turnover than regular milk.
 
UHT (Ultra-High-Temperature) pasteurization and aseptic packaging: 
This is sterile milk in sterile cardboard packaging, and it has a shelf-life of 3-12 months out of the refrigerator. The milk is pasteurized at 280° F - 302° F and then placed in sterile airtight containers in a sterile environment, destroying nearly all food-spoilage bacteria.
UHT milk is popular among campers, hikers, and people assembling emergency preparedness kits, but it has also entered the mainstream. Some single-serve flavored milks are produced this way because the longer shelf life is practical for use in cafeterias, vending machines, and sporting venues. 
 
Sources:
The  FDA Center for Food Safety A-Z Reference Guide
Steven C. Murphy, Senior Extension Associate (Milk Quality Improvement Program)
Department of Food Science, Cornell University
International Dairy Foods Association Factsheet: “Pasteurization--Definition and Methods”