It is divided into two groups:
1-Vertebrates 2-Invertebrates
Classification is all about organising living things into groups. The members of any group all possess a shared characteristic - it is this characteristic or feature that defines the group.
Taking the animal kingdom as an example, we can see that it is split into two clear groups:
Invertebrates - animals without a backbone.
Vertebrates - animals with a backbone.
The animals have been divided into two groups based on the presence or absence of a backbone. The backbone is the observable feature that defines whether the animal is a vertebrate or an invertebrate.
These groups are divided into smaller 'sub-groups'.
Sponges, corals, worms, insects, spiders and crabs are all sub-groups of the invertebrate group - they do not have a backbone.
Fish, reptiles, birds, amphibians and mammals are different sub-groups of vertebrates - they all have internal skeletons and backbones.
The animals that belong to these sub-groups all share the observable features of that group. Just as all the vertebrates have backbones, all birds have feathers and lay eggs, and all mammals have fur and suckle their young.
To see find out more about the observable features of these sub-groups and to see how they all fit together you can have a look at the relationship 'tree' - or what we like to call The tree of life.