A food chain is a linear order of organisms in which nutrients and energy flow as one organism eats another. The food chain levels are producers, primary consumers, higher-level consumers, and finally decomposers. These levels are used to describe the structure and dynamics of the ecosystem.
There is only one way through a food chain. Each organism in a food chain occupies a particular trophic (energy) level, its position in the food chain or web.
In various ecosystems, the base of the food chain consists of photosynthetic organisms (plants or phytoplankton) called producers. The organisms that producers consume are herbivores called primary consumers. Secondary consumers are usually carnivores that eat primary consumers. Tertiary consumers are carnivores that consume other carnivores. Higher-level consumers feed off each other lower trophic levels and so on up to the organisms at the top of the food chain.
An important limiting factor in a food chain is energy. Energy is lost at each trophic level and between trophic levels as heat and in transfer to decomposers. Therefore, after a finite number of trophic energy transfers, the amount of energy remaining in the food chain may not be large enough to support viable populations at higher trophic levels.
The linear model of ecosystems, the food chain, is a hypothetical and oversimplified representation of the ecosystem structure, such as a grasshopper feeds on leaves, a frog eats a grasshopper and gets eaten by a snake, the snake is eaten by an eagle, eagle dies and gets decomposed. This is an example of the food chain.
Food web is a concept that accounts for the multiple trophic (nutritional) interactions between each species. All the organisms are grouped into appropriate trophic levels, some of those organisms can feed on more than one trophic level. Species feed and are eaten by different species. This multi-complex network that contains many food chains is called a food web.
Each trophic level has less available energy and usually, but not always, supports a smaller mass of organisms to the next level.
A grassland food web has plants or other photosynthetic organisms at its base, followed by herbivores and several carnivores. Organisms that feed on decomposed organic materials decomposers (which decompose dead and decaying organisms) and detritivores (which consume organic waste). These organisms are usually bacteria, fungi, some invertebrates, etc, that are recycling the organic back to bio part of the ecosystem as they are as well eaten by others.