The difference can be confusing for the mere fact that you supplement your diet by taking vitamins, or you take supplements that contain vitamins. While the two like to intertwine, there is a small difference between what they are.

Vitamins

Vitamins technically fall under the supplement category. Like I mentioned above, you can supplement your diet with vitamins. These will generally come in pill-form, and sometimes in powdered form such as with some forms of vitamin C, like calcium ascorbate.

Some other examples of vitamins include:

  • Vitamin D
  • Folic acid (vitamin B9)
  • Vitamin B12
  • Vitamin A
  • Many more...

Another way of getting vitamins that many people don't realize is through fortified foods. Because many vitamins are hard to come by in food (vitamin D for example), there are many foods available that have been fortified with vitamins. Depending on the country and its food regulations, fortified foods may be more common in some areas of the world compared to others. 

Quote

Few foods are naturally rich in vitamin D, so the biggest dietary sources of vitamin D are fortified foods and vitamin supplements. Good sources include dairy products and breakfast cereals (both of which are fortified with vitamin D), and fatty fish such as salmon and tuna.
— Harvard.edu

Supplements

Supplements, on the other hand, are a wider variety of dietary products that may contain vitamins, minerals, herbs, and other nutrients. A good example of a supplement is creatine; it's produced in the body, but many people supplement with it due to the health and athletic benefits it provides. 

Some other examples of supplements include:

  • Fish Oil
  • Ginseng
  • Glutamine

The list goes on — there are far too many supplements to list here.

Vitamins nor Supplements Are FDA-backed

Supplements are not regulated by the Food and Drug Administration, contrary to what many may believe. This is due to the fact that, for things to be approved by the FDA, they need to be scientifically scrutinized to their standards. This isn't to say that science hasn't shown the potential of certain supplements, but just that they haven't quite met the standards of the FDA.

When purchasing a supplement, it's important to know the quality of the product you're purchasing. Buy from reputable companies that do third-party testing, as the FDA isn't the one checking your supplements to see if they're legitimately healthy for you.

A great source that does their own testing on popular brands is Labdoor. This site tests and ranks supplements and they are free of manufacturer biases. They even give science reports on individual products along with infographs.

labdoor-data-chart.png

Concluding...

To recap, a vitamin is something you can supplement and it falls under the supplement category. Supplements, however, are a wider variety of substances that are aimed at bettering one's health and don't always need to have vitamins in them.

A vitamin can be in the form of a supplement, but a supplement need not be a vitamin.

Always be wary of who you're buying products from and try to find the most reputable sources for your supplements. If something seems cheaper than the rest of its competitors, this should raise a red flag and you may want to look elsewhere. You are, after all, supplementing for health reasons — so buy products you know are good for your body.