Homogenized milk is milk that is one consistent mixture throughout. Milk that is not homogenized has a layer of cream(fat) floating on top of the watery liquid milk. Homogenization mixes everything so that it becomes a suspension. This is accomplished by breaking the fat into smaller particles so that it no longer separates. Milk that is skimmed has only .1-.3% fat content compared to whole milk. It is more popular in the U.S. as compared to Britain. It is more heart healthy as the fat content has essentially been removed. The fat in whole milk is saturated fat and this type of fat is known to be an unhealthy fat as related to cardiovascular disease.
Homogenized milk is milk that has been run through a process that breaks the large fat globules in the liquid into smaller fat globules. The globules of fat remain suspended in the milk. The milk is forced through a small valve under very high pressure which breaks apart the larger fat globules. The fat content of the milk hasn't changed.
There are some health concerns relating to homogenized milk due to the high fat content. A diet high in fat has been associated with arteriosclerosis and heart disease.
Skim milk is milk with a very low fat content. In some countries skim milk is labelled as fat free milk due to the very low fat content. Fat adds flavor to milk and some people don't care for skim milk because of the difference in taste.