Food chains and food webs and/or food networks describe the feeding relationships between species in a biotic community.
In other words, they show the transfer of material and energy from one species to another within an ecosystem.
As usually put, an organism is connected to another organism for which it is a source of food energy and material by an arrow representing the direction of biomass transfer.
Organisms are grouped into trophic levels based on how many links they are removed from the primary producers.
Primary producers, or autotrophs, are species capable of producing complex organic substances (essentially "food") from an energy source and inorganic materials.
These organisms are typically photosynthetic plants, bacteria or algae, but in rare cases, like those organisms forming the base of deep-sea vent food webs, can be chemotrophic.
All organisms that eat the autotrophs are called heterotrophs.
They get their energy by eating the producers.
A food chain describes a single pathway that energy and nutrients may follow in an ecosystem.
There is one organism per trophic level, and trophic levels are therefore easily defined.
They usually start with a primary producer and end with a top predator.
Here is an example of a food chain: phytoplankton to copepod to fish to squid to seal to orca.