A grasshopper is eaten by a frog which gets eaten by a snake and the snake is eaten by eagle. Thus, in the biosphere, one living organism is eaten or consumed by the other. This process of ‘who-eats-whom’ is called the food chain.
All the food chains start with plants, the producers of food. You know every well that food provides energy. Thus, we can say that in the biosphere, energy flows in the following direction.
Sun-producer-primary consumer-secondary consumer-decomposer
The process of transfer of energy from producers to consumers and finally to decomposers is called energy flow.
In nature, food chains do not occur in isolation. They are always interconnected. The network of interconnected food chains is called food web. This happens because one animal is food not for the one but several animals. For example, a grasshopper is eaten not just by frogs but also by birds, lizards and cats. As you know, a hervivore consumes many types of plants and many kinds of animals derive nutrition from herbivores. As a result many food chains are linked together to form a food web. Thus, a network of interconnected food chains is called a food web.