Kefir is a dairy product that results from a highly complex matrix of microorganisms fermenting milk. The array of microorganisms includes bacteria and yeasts which together form a matrix that shows up in the macro (where we can see its) as kefir grains, little coherent lumps that look almost like cooked tapioca beads. They are surrounded by polysaccarides and other components which have the appearance of stringy slightly thick mucous, and placed and mixed into milk will thicken the milk to a pourable yogurt consistency.
As the microbiota in the grains spread out and ferment the milk they produce lactic acid, carbon dioxide, polyscaccarides, and more. The acidity will curdle the milk somewhat and eventually begin to separate whey out of it if left long enough. The resulting kefir has a sour, sometimes slighly yeasty smell, and depending on the fermentation conditions it will likely taste also somewhat carbonated and have trace amounts of alcohol in it. Due to the carbonation it is sometimes referred to as “the champagne of dairy." Kefir is generally highly probiotic and beneficial for maintaining and restoring a healthy balance of intenstinal microbiota, so long as the kefir culture (grains) are maintained and fed regularly and kept in balance.
Remarkably, kefir's typical matrix of microorganisms have the property that they act together almost as one organism, mutually benefiting each other, and emitting anti-bacterial and other peptides that ward off undesirable other bacteria that would otherwise contaminate the kefir. Their functioning is analogous to an immune system, fighting off invaders. While it doesn't always work, it does help keep kefir cultures going for quite a long time when they are taken care of.
Kefir typically is fermented at room temperature, and is fed regularly (once or even twice per day), much like a sourdough starter or like a raw milk clabber. The amount of grains will grow in size over time, and it must be reduced, split, shared, etc in order to keep the proportion of grains to the amount of milk fermented roughly the same. When it gets out of balance due to fermentation temperature, feeding frequency, or contaminants, it may begin to smell overly yeasty or otherwise become less desirable.
Yemoos has extensive information on milk kefir, its maintenance and preservation, its benefits, and so on, and is a fantastic resource for learning more about it.
David Asher in his popular book The Art of Natural Cheesemaking advocates using kefir as a starter culture for cheese. While it seems like a good idea, this should be considered a more advanced technique due to the unpredictability of kefir cultures; they vary in their composition and balance even over time, and their hosting of yeast cultures can sometimes result in surprises in the vat or in the finished cheese. The taste of cheese made with kefir is sometimes distinctive and not at all expected, although many cheeses can be made with kefir to good effect and produce a decent cheese nonetheless. For those who want to experiment with alternative cultures for cheese making, first familiarize yourself with more predictable commercial cultures and their results, and then try clabber cultures or whey backslop techniques if you have an abundance of milk and can make cheese quite often. They will likely produce more consistent and predictable outcomes, and lay the foundation where experimenting with kefir cultures for cheese will be a more rewarding experience.