Quinoa is not a grass, but is grown as a cereal crop. It is more closely related to beets than wheat, and is a very good source of protein and nutrients. Quinoa seeds are usually eaten whole, cooked. The seeds have a bitter coating which needs to be removed before cooking.
Rice is the largest crop grown for food in the world. Most rice is grown in China and India. Rice is a type of grass that grows to about a metre high. Rice needs a lot of water to grow, and in countries like Australia that have a low rainfall people are calling for rice growing to stop.
Rice is usually eaten cooked plain, but can be prepared in a lot of different ways and served with lots of different foods. Rice can even be puffed, a little like popcorn.