The short answer to this question is that Entomology is the study of insects. All words ending in -ology are the study of something, as -ology is Greek for study. The rest of the word also comes from the Greek word gor six legged arthropods, as all insects are: six legs, three body parts (head, thorax and abdomen) and a jointed exoskeleton for support and protection. All insects share these fundamental characteristics but after that, the variations on the theme are so many and varied that the resulting study is virtually never-ending. Entomology encompasses the study of some one million described species and probably at least another 2 million as yet unnamed species. As well as naming new species, the subject includes all the information about the individual species, from anatomy and physiology to life styles and life cycles. We know almost nothing about most species and each species could be a lifetime's study for a single entomologist.
This is a field in which anyone with an interest in wildlife can enjoy and make a contribution. Many people collect insects, especially butterflies, as a hobby. A great many more people enjoy observing and photographing insects. In addition there are many paid jobs as entomologists because so many insects are important economically. Some species are beneficial, such as bees and silkworms, which produce valued crops. Others are significant agricultural pests that do immense damage to crops and stored agricultural products. Entomologists are at the forefront of the battle against such pests. Without basic knowledge about these animals, we cannot successfully fight them or learn to live with them.
The study of insects is also important in medicine because so many diseases are spread by insect vectors. Malaria is an excellent example because the disease has taken so many lives and the fight against the disease-carrying mosquitoes has had such devastating consequences due to inadequate knowledge before important decisions were made. The decision to use DDT so widely nearly wiped out dozens of other species and was a significant source of chemical pollution in many ecosystems because the effects of the chemical were not adequately studied before widespread use.
Other insects are important for research. What would the study of genetics be without the humble fruit fly? These little animals have contributed much to our understanding of the mechanisms of inheritance.
What does it take to become an Entomologist? A college degree is a start. Many top jobs require a PhD, but there are also Technical positions available. Research positions are available at universities and in government departments such as the Department of Agriculture. Some private companies also fund research into economically important species.
If you are interested in insects, you can study them as a hobby. Even the most common species have undiscovered secrets. Sit in the garden and observe the butterflies, the bees and the grasshoppers. There is so much to be learned.