Have to crash in on this - curd cheese is similar to cream cheese but with a lower fat content.
It makes a slightly more acidic contribution to a recipe than cream cheese, and another substitute is greek yoghurt. Use it in souffles, cheesecakes, crowdie, yorkshire tart....etc
Quark is an unaged cheese mostly sold in the UK without the whey present, but some continental varieties are 'wetter' and have more of the liquid present - depends on the brand. It is actually one of the better ingredients to use in a cheesecake recipe....you can make your own quark cheese - if you have 2 days and fresh milk to hand, but the bought will give good results.
Hope this helps..oh, and if you are worried about the fat content of the meal - serve the cheesecake with lashings of clotted cream on the side.....!