MILK CREAM
If we take a can of fresh milk and leave it to rest for some hours, we'll soon see some fatty substance form on the surface of the liquid. This is simply the milk fat globules that make their way to the top, due to their low weight. This layer of fatty milk can be skimmed off the surface and there we have a nice 20% cream. People usually love cream for its velvety texture and taste, without a greasy feeling. All this can be ascribed to the small fat globules that are finely dispersed in water. The concentration of this fat is nowadays industrially "adjusted" - we can buy lighter or heavier creams, e.g. half-and-half is about 12%, while heavy cream gives us about 40% of fat. The different fat content determines what we can use them for. Lighter creams are popular as additives in coffee or tea and lighter sauces, while heavy cream can be used in thick sauces or it can also be whipped into a foam. The fact that lighter creams cannot be whipped and can curdle in sauces (unless you buy those super creams that don't curdle in heat!) makes heavy cream a more flexible alternative. You can always dilute heavy cream with milk to the desired consistency or fat content.
What has the fat to do with curdling of light cream? When heating the cream, milk proteins become destabilized and can fall out, especially in sour or salty dishes (like a nice sauce usually is). It seems that the more fat we have in our milky fluid, the better chance there is to protect the milk proteins from curdling. Fat simply adheres to the proteins and creates a shield that prevents the proteins from attaching to each other and form the white flakes.
Let us also bring up the subject for which most people love cream - the whipped cream. Due to a high fat content in heavy cream, the air can be beaten into the liquid and the small air bubbles can become trapped within a network of fat. The fat network is also created by whipping: as we beat the cream vigorously, the fat globules become unstable and start to attach to one another, as the proteins around the globules become detached from the fat. An air bubble can be trapped between the walls of fat just because the fat molecules cannot be stabilized if they don't face a surface that is not fat. So the fat network is formed and the air pockets are beaten into it. Even if there still is a lot of non-fat fluid that could destroy the entire foam, the fats are so well brought together that the problem simply doesn't happen. If we continue to beat the cream for a longer time, the fat network starts to collapse, because the force of attraction between the fatty threads becomes too big to retain the air pockets in it. A kind of buttery grains are formed.
Some tricks for whipping cream? One, whipping can be made faster by dismantling some of the proteins that protect the fat globules from aggregating. It can be easily done by adding a small amount of acid, e.g. lemon juice, into the cream. Two, make sure that the cream you are whipping is really fat enough - it should be above 30%. Three, make sure that the cream is cold, so that the fat doesn't melt!