Can I use cream of tartar in place of tataric acid
While normally I would hesitate to recommend this source, eHow has a good article on the difference.   Cream of tartar is ground, partially neutralized tartaric acid, and not generally a substitute.

In the end, they recommend:


  If your recipe calls for tartaric acid and you don't have it, using
  cream of tartar might work. For every teaspoon of tartaric acid,
  replace with two teaspoons of cream of tartar. However, using tartaric
  acid will produce better results.


However, there are very, very few applications of cream of tartar in cooking.  The most prominent is encouraging foaming of egg whites.   With good technique and patience in whipping the whites, it can simply be omitted with good results.

It is also used in some candy making, to interfere with crystal formation in sugar work.  In this application, substitution may actually be effective, but you might wish to use alternatives such as invert sugars or citric acid.

In the specific jam recipe, if you are canning, one purpose of the tartaric acid is to ensure the canned goods are sufficiently acidic as to prevent the growth of botulism, and keep your put up green tomatoes safe.  In this context, you should not substitute.  When canning, always follow trusted recipes precisely, to ensure a safe outcome.

Even if you are making refrigerator jam, this is not a good substitution candidate, since you may need the acidity to help get a good gel.   You might be able to use citric acid (I am not sure how the acidity levels compare), but don't try this if you are canning.