Rava is the Malayalam term for Semolina which is also called Soji or Sooji in other parts of India.
Payasam is referred to as Kheer in other parts of India. It often made from Rice, Semolina, Broken Wheat, Oats, Vermicelli and Cheru Payaru.
Semolina or Rava is called Griess or Grieß in German which is prepared usually as Griessbrei/Grießbrei (Semolina Pudding/Semolina Porridge) in Germany. As far as I can remember, we never use Semolina in the Philippines to make sweets or desserts.
The picture below is a very simple Rava Payasam which was cooked plain. The fancy Payasam which is a part of Onam Sadya is usually with Cashew Nuts and Raisins.
Rasam refers to the sour, spicy and tangy clear/thin soup from Kerala, India using basic ingredients, among which is the Tamarind (Tamarind Paste) which gives Rasam its sour taste. Tomatoes and Lemon are also used to make Rasam taste sour. Other ingredients and spices used for making Rasam are onions, ginger, garlic, turmeric powder, mustard seeds, pepper, red chilies and a bit of Jaggery. Other versions make use of Cumin, Coriander and of course the Curry Leaves which seems to be a must in all the dishes from Kerala.
Rasam is a popular dish not only in Kerala, but in other states of South India. It is often poured over rice and combined well and eaten also as a side dish together with Meen (Fish) Curry. It can also be served as a soup.
Rasam is also used as a digestive drink and is one of the many dishes serve as part of Sadya.
This Rasam below is made sour by using Tomatoes. In my opinion the South Indian Rasam is very much the same as the Philippines Sinigang, except that the Philippines Sinigang has meat (fish, shrimps/prawns, pork or beef) on it. (Please see article on Sinigang)
Thakkali (Tomato) Rasam poured to over rice, sometimes the only meal for lunch or dinner
Below is a picture of the traditional Rasam. Other varieties are coming out now using other ingredients.
Rusks are pieces of white bread which are dried and baked until golden brown. In Kerala, India, particularly in Allepey, they can be bought in most variety stores and eaten as snacks with hot coffee or tea or made into breadcrumbs as coating for Cutlets (fish or meat cutlets) before frying, as in Japanese Panko.
Picture below is a picture of store bought Rusks and pieces of Fish Cutlets coated with breadcrumbs made from Rusks.
Fried Fish Cutlet from Allepey, Kerala, India usually coated with flour, but this time Rusks made into breadcrumbs was used as a coating for extra crunchy taste