The food chain in a visual representation of how energy is flowed through out a specific environment. While it is very similar to the food web, the main difference is that the web gives a more broad wide range of organisms and it is more specific with what eats what. The second part of your question about a few animals roles umm... Grass is a producer which has the role of feeding primary consumers, it is the base of the chain or web without it the whole thing would collapse. Tertiary consumers such as owls have the role of eating secondary consumers. I always thought of it like the tertiary consumers are population control since nothing eats them they have no responsibility to feed other organisms just make sure that secondary consumers don't overpopulate.
