Cream of tartar is not the same thing as meringue powder, and they have different purposes. However, meringue powder often includes cream of tartar as one of its ingredients.
Meringue powder is typically used to create royal icing. When water is added to meringue powder and whipped vigorously, meringue is formed. The ingredients of meringue powder include dried egg white, sugar, cornstarch, gums and stabilizers.
Cream of tartar is typically used as a stabilizing agent or as an acid ingredient to activate baking soda. It is a byproduct of winemaking and is also known as potassium bitartrate or potassium hydrogen tartrate.