There is no shortage of trendy yoghurts at the grocery store these days, from quark to labneh, but not all cultured milk is created the same. Most of these foods have existed for as many as thousands of years, and were often methods of preserving milk. Milk kefir, commonly just called kefir, is one example. Originally from Eastern Europe, kefir has gained popularity in the US and is now commercially available at many grocery stores. But to think of kefir as just "drinkable yoghurt" is only half of the story.
Like yoghurt, kefir is made by adding live cultures to milk, Unlike yoghurt, that is done by adding kefir grains (sort of pebbles of active yeast cultures) to milk. You can even buy your own grains online. Kefir also is created at room temperature, whereas yoghurts are heated. The process can even give it a slightly bubble flavour. It's also more sour than yoghurt, though devotees like the taste. If you're afraid of plain kefir, many commercial brands sell flavours and even find ways to sell it in cups to make it more like its more familiar cousin. Kefir's unique cooking process also makes it 99% lactose-free and easier to digest for people with lactose intolerance.
If you're intrigued but unsure about trying kefir, the flavoured options can be a great way to go. Still too much? Consider adding it to things you'd also put yoghurt in: with granola or fruit, or in a smoothie. You could be buying your own kefir grains in no time.