Is tartaric acid the same as citric acid?  I meant to buy tartaric acid and bought C acid by mistake.

No, it isn't the same!  :)

but can you use on instead of the other in a recipe?  I was told they did the same thing. Is this correct?

I wouldn't think so, what are you cooking?

Just 'googled' it and it seems that you can substitute Cream of Tartar with 1/4 teaspoon of lemon juice for each 1/8 teaspoon of Cream of Tartar.  If the recipe calls for baking powder combined with  cream of tartar, you can use just baking powder but subsidising the cream of tartar with the same amount of baking powder in other words, you'll be using more baking powder to replace the C of T.  :)

tartaric acid and citric acid are not ( I think) anywhere near cream or tartar or baking powder.  I know that you have to sign for citric acid in the chemist as seemingly drug users use this to make there chosen drug go further.