Quark is a dairy product that is made from soured milk. It is warmed until curdling occurs, then strained. It is a fresh product of white color that usually does not contain salt.
Greatest popularity of quark is in Central, Northern and Eastern European countries.
It is distinct from curd or cottage cheese and it should not be replaced with each other in recipes.
On the west coast it can be found in few stores like Metropolitan Market. You can also find it at German Delis in the area. It is a product locally made by Appel Farms. Here is the direct link to their web page.