Allotropes of carbon includes substances such as graphite, diamond or buckminsterfullerene.
They’re all similar in the thing that they’re all made out of carbon only.
However, their structure is different, such as graphite has a layer structure, diamond has a tetrahedral structure, and buckminsterfullerene has a spherical structure.
Since they have different structures, they have different physical properties too. For example, diamond is hard because all the carbon atoms in the structure is held together by strong covalent bonds, while graphite are graphene layers that are held by weak intermolecular forces which makes the layers slide over each other easily thus making graphite soft.