It's like yoghurt but a bit thicker. Not as tart either but still needs some sort of seasoning! Quark is like Philadelphia with all of the fun taken out, very dry and bitter, needs lots of seasoning. Both can be used in sweet or savoury dishes.
i use ff natural fromage frais a lot, i prefer it as its a bit thicker than natural yogurt, i like to put a big spoonful on my plate with chilli, or if im having some spicy coated veg it helps cool my mouth down
I like the ff natural greek yoghurt with fruit, its lovely. Quark is only really good to cook with, as it has pretty much no flavour. It's good to use with a couple of laughing cow light cheese triangles for a cheesy kind of sauce but its not as smooth. I use it to stuff mushrooms, mixed with bacon and leek and laughing cow light and a teeny bit of parmesan on top