Trophic level is an organisms position in a food chain (or food web), and describes the way energy is obtained. (Purves, 1060). For example, plants are in the first trophic level. As producers who make their own energy, they are known as autotroph. Hetertrophs that eat plants (herbivores) are in second trophic level, and carnivores follow.
When one animal eats another animal, the energy is lost. When the consumed animal moves, it loses additional energy. As the trophic level goes up, loss of energy in the process goes up. 
In the ecosystem, there are four basic components: abiotic environment, producers, consumers, and decomposers. Plants are known as producers because of their production of chemical potential energy through photosynthesis. Consumers must eat other organisms to obtain their energy. Carnivores and herbivores are both consumers, which are otherwise known as heterotrophs. Decomposers (i.e. scavengers and detritivores) eat dead organisms or waste products.(Purves, 1060)
Heterotrophs are the organisms that depend on other organisms for their nutritional requirements. (Hetero means other and troph means nutrition in Greek).  Animals, fungi, bacteria, and non-photosynthetic organisms are the examples of the heterotrophs.(Purves, 1060)
The heterotrophs need to get organic compounds either directly or indirectly by ingesting other organisms or their products.(Purves, 620) Therefore, heterotrophs occupy the consumer trophic level. They consume the primary producers first compare to other consumers, so they are called primary consumers. If the heterotrophic organisms consume plants, they are called herbivores. When organisms consume the herbivores, they are called second consumers. Tertiary consumers eat the second consumers and so on. (Purves, 1060)
Most plants are autotrophs, but some live as obligate parasites and are therefore heterotrophs. If an organisms cannot synthesize the organic and inorganic compounds they need to survive, but must obtain them by consuming other organisms, they are considered heterotrophs. Depending on how the heterotrophs use carbon, there are two subtypes of the heterotrophs.
Chemoheterotrophic organisms must obtain organic building blocks, because they cannot produce those by themselves. They gain the energy from the organic molecules. Most archaea, bacteria, animals, and fungi are the examples of chemoheterotrophs.(Purves, 530)
Photoheterotrophic organisms use the sun’s light as the energy like other phototrophic organisms, but they gain carbon by consuming other organisms. The examples of the photoheterotrophic organisms are purple non-sulfur bacteria. (Purves, 530)
Most plants are the autotrophs, but some plants live as the parasitic lives. Therefore, these kinds of the plants are the heterotrophs. The heterotrophs cannot synthesize the organic and inorganic compounds as the autotrophs. They consume the autotrophs or other heterotrophs to get energy. The heterotrophs have carbon from the organic compounds. 
Autotrophs are the organisms that produce complex organic compounds using energy that is either derived from the sun or inorganic chemical reactions.(Auto means self and troph means nutrition in Greek).
Usually, producers occupy the autotrophic level. The autotrophs are the most important organism in an ecosystem. They obtain nitrogen from other organisms but not energy. The plants or the autotrophs use sources from the environment such as the sunlight and inorganic compounds to live. They become important resources for the heterotrophs. (Purves, 717)
Photoautotrophs collect energy from the sun and make energy through photosynthesis.(Purves, 962) Because they gain energy by photosynthesis, they are called photosynthesizers. Primary producers are another name to the autotrophs, because they are in first position in the trophic level.(Purves, 1060)
Phototrophic organisms perform photosynthesis to gain their energy. (Photo means light and troph means nutrition in Greek.) (Purves, 530) Aquatic organisms are examples of phototrophic organisms. They produce starch to use when the organisms do not have much energy to conserve. 
Chemotrophs (or lithotrophs) are the organisms that gain the energy by consuming the organic molecules or carbon in an organic form. Explaining the chemotrophs further more, the chemotrophic organisms gain the energy by the oxidation of various molecules. (Chemo means chemical and troph means nutrition in Greek). They can be the autotrophic or the heterotrophic. 
Chemoautotrophic (or chemolithotrophic) organisms gain the energy from the chemical reactions by synthesizing the chemical compounds. They use only inorganic energy sources, while the chemoheterotrophic organisms use the organic sources.  The chemoautotrophic organisms use photosynthetic cycle or other pathways to fix carbon dioxide. Some organisms oxidize ammonia or nitrite ions to nitrate ions, and some oxidize hydrogen gas, hydrogen sulfide, and sulfur. Most archaea are the chemoautotrophs. Chemoautotrophic prokaryotes are base ecosystem in deep sea. (Purves, 530)
The trophic level happens in sequence, so it can use the ecological pyramid to explain it easily.  There are three types of the pyramids and each name describes what kinds of the pyramids are.
Numbered pyramid shows the number of the organisms in each trophic level. There are more small animals than large animals. As the pyramid goes up, the number of the organisms is less than the organisms in the lower level. 
Biomass pyramid shows total number of existing organisms in each trophic level. (Purves, 1060) Total mass of the consumers is less than total mass of the predators. Products of the consumers are not important as the those of the producers, so the number of the consumers is not much. Total mass of the predators is less than total mass of the herbivores with same reason. For example, there are great amount of grains than the mice to support the mice. Greater number of the mice supports less number of the hawks. 
Energy pyramid shows decreased energy from low trophic level to high trophic level. (Purves, 1060) When the trophic level goes to the higher level from the lower level, the energy is lost and decreasing waste. Also, the potential energy changes into the kinetic energy and the heat energy as the energy pyramid goes up. Because the energy pyramid can compare different ecosystems, it is used often more than other pyramids. 
When more than two trophic levels are involved, it is called the multitrophic interactions. Usually this term is used to an interaction among the plants (producers), herbivores (consumers), and carnivores (consumers). The producers in the first trophic level are eaten by the consumers in the second trohpic level. Strong and bigger consumers in the third trophic level (sometimes in the last trophic level) consume the weak and smaller consumers. 
Omnivores consume both plants (autotrophic) and animals (heterotrophic) organisms, and are therefore considered multitrophic. Many species commonly referred to as carnivores are actually omnivores.(Purves, 1060) Examples of the omnivores include dogs, cats, and seals.
- Autotroph Wikipedia
- Chemotroph Wikipedia
- Heterotrophs Wikipedia
- Trophic Level Wikipedia
- Trophic Levels and Food Chains University of Wisconsin
- Purves, William. Life the Science of Biology. 2005. Couriers Company Inc.