Kefir is a drink that you can make at home from milk. It looks like yogurt, or a yogurt drink, and it has existed in many cultures for a very long time.
Kefir (a probiotic) is similar to yogurt (a prebiotic); however, it is vastly more powerful. Kefir will aggressively populate your digestive system with many beneficial organisms that will kill pathogens in your digestive system, and thus strengthen your immune system. It is also a great source of nutrition.
Kefir is touted as a cure for many different ailments, and if you search the web you will find people using it to help with cancer and chemo. An example of a technical explanation of kefir vs. cancer:
I would not buy kefir products in a store because they are probably not strong enough to help. So instead, I make it. I make it so that I know it is strong and unprocessed and untarnished by any additives. I make it by fermenting organic milk with kefir grains. The longer you ferment it, the stronger it is. I make some daily, as the process takes about 24 hours to reach full strength.
Finding kefir grains is difficult, so don’t expect to see them in your local supermarket. Most people get them from friends or online specialty shops. There is not much business incentive in selling the grains, so most companies sell powdered versions that you have to keep buying. But if you get the grains, you just keep reusing them.
You can drink kefir in the form of smoothies made with bananas, berries, or whatever you can dream up.
Here is a very informative site for reading more about kefir.
While the above site is excellent for information, I do not recommend their products at all, as they do not provide the grains.