Food chains show the eating patterns in an ecosystem. Food energy flows from one organism to another. Arrows are used to show the feeding relationship between the animals.
The producers are always at the beginning of the food chain, bringing energy into the ecosystem. Through photosynthesis, the producers create their own food in the form of glucose, but also create the food for the other organisms in the ecosystem. The herbivores come next, then the carnivores. When these consumers eat other organisms, they use the glucose in those organisms for energy.
A food web shows the feeding relationships between many organisms in an ecosystem. If you expand our original example of a food chain, you could add deer that eat clover and foxes that hunt chipmunks.
A food chain is a diagram that shows feeding interactions in an ecosystem through a single pathway.
A food web is a diagram that shows feeding interactions between many organisms in an ecosystem through multiple intersecting pathways.