Keshav Bhatia, Academic Content Writer at Edumarz

Similarities among organisms allow us to group them and then study the groups.

A characteristic of an organism is the detail of its appearance or behaviour, in other words, a particular form of function.

Example; mammals produce milk and plants can photosynthesize
Some basic characteristics are shared by all organisms, like all life is cellular, we do not consider viruses alive
Then a distinction is made between prokaryotic and eukaryotic organisms
Prokaryotes do not have a nucleus whilst eukaryotes do
And then there’s more classification based on cell number and organization, some organisms are multicellular whilst some are unicellular.
We can make more classification based on whether these multicellular organisms can perform photosynthesis or not, or if they have the ability to move.

The more characteristics two species have in common the more closely they are related, and the more closely they are related, the more recently they will have had a common ancestor.
Classification of a species reflects their evolutionary relationships.

If we keep going on and on building up groups and supergroups of species with common ancestors and common ancestors of these common ancestors we will reach a single species at the beginning of life or at the beginning of evolutionary time.

Identifying common characteristics and tracing evolutionary relationships

Mammals, birds, reptiles and amphibians all have four limbs. The basic structure of the limb is similar, though it has been modified in each group to perform different functions.
These organs are called homologous organs.
Birds and bats both have wings and use them to fly, but that does not imply that their wings developed from a common origin, because the wings of a bat are simply folds of skin stretched between elongated fingers, whereas the wings of a bird are feathery coverings all along the arm.  

These types of organs are called analogous organs.