When unhomogenized milk is left to stand it separates. The thick stuff on top is the cream. Heavy cream is the richest type of liquid cream with a fat content of at least 36%. (Whipping cream ranges between 30% and 36%.) Because of its high fat content, both heavy and whipping cream whip fast, with heavy cream slightly more firm). Heavy cream is also less prone to curdling when reduced, making for thick, velvety, rich sauces.
Most cream comes ultra-pasteurized, meaning it's brought to a higher temperature for a shorter period of time than regular pasteurized cream and milk. This extends the shelf life (before opening) of milk and cream under refrigerated conditions. Regular pasteurized cream, however, can give you thicker results. Check the sell-by date before purchasing.
Always taste cream before using to be sure it still tastes fresh.