While both Kefir and yogurt are cultured milk products, they contain different types of beneficial bacteria. Yogurt contains transient beneficial bacteria that keep your digestive system clean and provide food for the friendly bacteria that already are present. Kefir actually helps to colonize your intestinal tract -- a feat that yogurt cannot match.
Additionally, 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 help balance the intestinal flora, including promotion of beneficial yeast in the body by penetrating the mucosal lining. They form a virtual SWAT team that housecleans and helps strengthen the intestines.
Kefir's active yeast and bacteria may provide more nutritive value than yogurt by helping digest the foods that you eat and by keeping the colon environment clean and healthy. The curd size of Kefir is smaller than yogurt, so it's also easier to digest, making it an ideal food for babies**, the elderly, and anyone with digestive health concerns.
My own recommend website for kefir: