They are both cultured milk products but they contain different types of beneficial bacteria. Yogurt contains transient beneficial bacteria that keep the digestive system clean and provide food for the friendly bacteria that reside there.  But kefir can actually colonize the intestinal tract, a feat that yogurt cannot match. 

Kefir contains several major strains of friendly bacteria not commonly found in yogurt, Lactobacillus Caucasus, Leuconostoc, Acetobacter species, and Streptococcus species.

It also contains beneficial yeasts, such as Saccharomyces kefir and Torula kefir, which dominate, control and eliminate destructive pathogenic yeasts in the body. They do so by penetrating the mucosal lining where unhealthy yeast and bacteria reside, forming a virtual SWAT team that housecleans and strengthens the intestines. Hence, the body becomes more efficient in resisting such pathogens as E. coli and intestinal parasites. 

Kefir's active yeast and bacteria provide more nutritive value than yogurt by helping digest the foods that you eat and by keeping the colon environment clean and healthy.

Because the curd size of kefir is smaller than yogurt, it is also easier to digest, which makes it a particularly excellent, nutritious food for babies, invalids and the elderly, as well as a remedy for digestive disorders.

So today I bought some and made a great fruit smoothie with it.