Curd and Yogurt-Dahi or curd is the Indian variety, a version of yogurt and is widely used in the daily menu  of an  average Indian. Several organisms are involved in  the  preparation of Curd which contains lactic  acid. Yogurt  is  the  preparation with the action  of  two organisms,  Streptococcus thermophilus  and  Lactobacilius bulgaricus.  The  ideal ratio  of  these two organisms is  1:  1  for best results. In common usage,  Curd  refers to  'the  domestically prepared fermented milk  using  a  starter of the previous  day' while yogurt refers to  the  'industrially produced fermented milk using a particular bacteria  as  the  starter.'