1) Producers can make their own food by capturing the sun's energy, but consumers and decomposers can't. Consumers need to eat other organisms to obtain energy. Producers allow energy enter in the food chain. They are generally green plants.
2) Decomposers and scavengers break down dead plants and animals. They also break down the waste of other organisms. Decomposers are very important for any ecosystem. If they weren't in the ecosystem, the plants would not get essential nutrients, and dead matter and waste would pile up.
3) Energy enters in a food chain in the form of solar energy (sunlight) and leaves in the form of heat.
(p.s.I too read it randomly on some sites....)