Animals can be classified into two main groups: vertebrates and invertebrates. Vertebrates have a backbone and include animals like mammals, birds, reptiles, amphibians, and fish. These animals often have complex structures and systems, allowing them to move and interact with their environment in various ways. Invertebrates, on the other hand, do not have a backbone. This group includes insects, spiders, crustaceans, and mollusks. Invertebrates are incredibly diverse and can be found in almost every habitat on Earth, from the deepest oceans to the highest mountains. They play essential roles in ecosystems, such as pollination and decomposition.