Relationships between different organisms can be established from observable characteristics and translated into the form of a phylogenetic tree.
To do this, we must first choose comparable characteristics between the different living beings that we want to classify and then compare them.
Then, for each character, we determine whether it is an ancestral form or a derived (or evolved) form.
This is done by looking at the characteristics of the species that are known to have no derived characteristics among those chosen. This species is the outgroup.
Finally, a simple rule is applied: The larger the number of derived traits in common, the closer two species are. These similarities indicate that these species descend from a common ancestor.  

These species are therefore grouped together in groups called clades or taxa where all the species have the same common ancestor defined by the appearance of a derived trait.
A phylogenetic tree also makes it possible to tell an evolutionary story since the derived characters appear one after the other over time.