Food chain includes different species, each with a different specific tropic level. Only one type of species consumes the species in the previous tropic level. Meanwhile, food web includes several food chains, interacting with each other. One type of species can have a few different types of consumers, and each species can also be in several tropic levels at the same time.
Only around 8% to 10% of energy is conserved between each trophic level in a food chain. This is because organisms tend to loose most of its energy through life processes like excretion or movement, or also because there are some parts of organisms that are un-digestable, such as hair.