Germany’s Käsekuchen is traditionally made with quark rather than cream cheese.
German Käsekuchen is a baked cheese cake whose filling is usually based on quark, a fresh, acid-set dairy product with a smooth but slightly crumbly texture. It is commonly combined with eggs, sugar, and milk, then baked in a pastry shell or on a shortcrust or yeast-based foundation. Some versions are crustless.
This differs from the classic American image of cheesecake, which often uses cream cheese and a cookie-crumb crust. Quark is not the same as American-style cream cheese: it is generally less fatty, more tangy, and made by acidifying milk and draining away whey. German bakers may use different fat levels of quark depending on the desired richness and texture.
Käsekuchen has many regional variations, including fruit, raisins, vanilla, and streusel. The broader cheesecake family is international: Austria uses Topfen in similar cakes, Greece has versions using cheeses such as myzithra, and Sweden is associated with ostkaka. Those similarities do not change Germany’s canonical association in this question.