Difference between phenanthrene and anthracene?

Anthracene and Phenanthrene are isomers; they have the same molecular formula but different structural formula. You see, even the position and orientation of any group attached to a compound changes the characterstics of the whole compound

They are different compounds with different chemical and physical properties.

I’ll give you another example of structural isomers : n-Hexane and 2,3-dimethyl butane are two different compounds with different properties but they have same number of carbons(6) and hydrogens(14).