The Sun is the ultimate source of energy for all organisms.
Plants are producers, because they make their own food. Part of the energy they make this way is transferred to the herbivore animals that eat plants, the primary consumers.
Secondary consumers get their energy by eating the herbivores and they are called carnivores, because they eat other animals.
There are also tertiary consumers (and yes you guessed right... they eat secondary consumers!). Some animals eat both plants and animals (like humans) and they are called omnivores.
The feeding relationships described above are shown through food webs and food chains, which in turn can be shown with the use of pyramids of numbers and biomass.
Not much of the energy made initially from plants is transferred to animals. A lot of it is lost during the transfer and by the daily functions of organisms.
Some elements like carbon and nitrogen recycle in the environment with all this eating!
Let's explore all that in more detail in the questions that follow.