A consumer in an ecosystem is an organism that obtains energy by consuming other organisms, as opposed to producing it through photosynthesis like producers. Consumers can be categorized into various levels based on their dietary habits: primary consumers feed on plants (herbivores), secondary consumers eat herbivores (carnivores), and tertiary consumers prey on other carnivores. They play a critical role in food webs, influencing population dynamics and energy flow within ecosystems, and are essential for maintaining ecological balance.