Its very low fat cream cheese. I get mine in the cheese section at Tesco. It's a white rectangular tub with a blue label on the top. Usually blink and you will miss it. Some people add sweetner too it and it like that.
Quark is a really low fat cheese and when made in the recipes is really nice. It can be found in fridge in the cheese section. It's usually a a tub abit like cream cheese with the foil cover. Not expensive either about 60p for a 250g tub.
Just thought I would add that I tried to make a creamy mushroom soup with quark and it just ended up very grainy and tasted awful! Maybe I over heated it? If anyone knows why this happened it would be great to know.