As far as yogurt vs Kefir, many websites are either too technical or perhaps biased.. but I found one that seems to be a good and simple reference. This exerpt is taken from this site:
http://www.aldo.com/sgt/TesslerOnKefir2.htm
You might want to read the whole article or similar articles to get a clearer picture.
"Yogurt is made by culturing fresh milk (previously boiled and cooled) with a bacteria starter. The starter, usually containing two or three beneficial bacteria, comes from a previously made batch of yogurt. Yogurt must ferment in an environment warmer than normal room temperature (about 110 degrees), for a fairly specific amount of time (about 8 to 16 hours), in order to develop properly. The culture bacteria in yogurt have only a limited lifespan and must be renewed regularly with new culture.
In contrast, kefir is made by culturing fresh milk with live kefir grains. These grains are actually colonies of more than 30 bacteria and yeasts that are bound together in a stable, symbiotic relationship.[2],[3] Kefir ferments at room temperature, and has a wider range of acceptable fermentation periods (from about 12 hours to two days). Kefir grains stay alive and robust indefinitely. They also grow in size and number during the fermentation process. After the milk is fermented, the grains are filtered out and added to new milk for another batch of kefir."