Curd is made out of Buffalo milk and is in a sense similar with the consistency of Greek Yoghurt. This article http://www.lankanewspapers.com/news/2008/10/33756_space.html gives you some more information on.
As a Sri Lankan I love Curd, and consume a vast quantity of it while I am in Sri Lanka. It is in my books one of the best local deserts. Curd can be had with sugar, on its own or with Kitul Treacle. The most well known and accepted ways to eat curd is with Kithul Treacle.
Treacle is very similar to maple syrup, in a way both come from plants. Kithul comes from the sap of the Kithul Tree, which is a variety of Palm more information on this tree can be found here http://en.wikipedia.org/wiki/Caryota_urens .
Overall Nick has done a great job in the below video in describing this amazing Sri Lankan treasure. It is highly recommended to watch the below video and to try some curd (if you have no milk allergies) when you go to Sri Lanka.
