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.