Scientists like to be organised. So, that means that all living things are grouped according to features which we can see (and some we can't!).
To start with, organisms are grouped as plants or animals - it makes humans feel 'comfortable' to have them sorted out!
Then, animals are divided into two groups: those with a backbone, the vertebrates:
and those without, the invertebrates:
Vertebrates have a hard skeleton inside their bodies which gives them their shape.
These animals are grouped as fish, amphibians, reptiles, birds and mammals.
Are you ready to have a go at sorting things out?