The only important fact you should know between Kefir grains and yogurt starters is that the former has more beneficial bacteria made during the fermentation than yogurt starters. The process of making Kefir and yogurt from kefir starters and yogurts starters is basically the same. Their tastes are different because of the type of bacteria used. Kefir is not as thick, and has a buttermilk-like flavor. Both also contain very little amounts of alcohol. Yogurt starters need to be conducted in a warmer temperature, and is faster to make into yogurt than Kefir grains into Kefir drink. Yogurt makers are a huge help especially when making large batches.
The significant difference, however, is the bacteria in both products. You can get a lot more benefits from drinking Kefir than drinking yogurt. Yogurt keeps the digestive system clean, but Kefir can actually colonize your intestinal tract long term.
For more information, check out my posts on information on kefir.