Rabri is thickened sweetened milk having layers of malai or cream in it. it's flavoured with cardamoms, saffron with the addition of dry fruits like almonds and pistachios. This version of rabri is also called as Lachha rabri in hindi.
cooking milk till half of its volume to make rabdi recipe. You can also just thicken and scrape the milk solids from the sides like I have done for shahi tukda rabdi. but in this case you won't get the layers of cream or malai in the rabri. But both versions taste good.