Reviewing the Basic Definitions lesson may help with some of the terminology in this lesson.
We've seen how our bodies digest and use carbohydrates and protein. We've seen at a below cellular level what nutrients are doing for us. These two nutrients also depend on vitamins to work.
We would be hard-pressed to find someone who didn't know what vitamins are. Unfortunately, vitamins have been relegated to a substance found in a pill. We are vaguely aware that citrus fruits have vitamin C and carrots help your eyes because of some vitamin in them. It would be better to think of vitamins as a substance that encourage the processes we have discussed so far. In fact, some of what we have discussed would be impossible without them.
When vitamins were being "discovered," there were five and they were named according to the order they were discovered, A-E. As research improved, vitamin B was seen to be not just one vitamin, but 12 different substances. This is how we got what are referred to as "the B vitamins," and they technically all can be referred to by their number. Most people will use their adopted name to refer to them (Thamin, Riboflavin, Niacin, and Folate, to name a few). In addition, there is now another common vitamin referred to as vitamin K. Currently, the "official" requirement to be called a vitamin refers to if it is essential for life. I take issue with this, and I think future research will prove this definition as terribly limiting for nutrients of this sort.
To say a vitamin is water-soluble or fat-soluble refers roughly to how the vitamin is transported and absorbed, among other things. This will be covered in detail in future lessons.