Two structures that have the same atom-to-atom connections may be configurational diastereomers, configurational enantiomers, or homomers. (Two homomers may differ in their conformation, of course.)
Two compounds that have the same configuration at all of their stereocenters are homomers.
Two compounds that differ at some but not all of their sp3 stereocenters or at any of their sp2 stereocenters are configurational diastereomers.
Two compounds that differ at all of their sp3 stereocenters and at none of their sp2 stereocenters are configurational enantiomers (if they are chiral) or homomers (if they are achiral).
NOTE: Allenes and iminoketenes, R2C=C=CR2 and R2C=C=NR, in which the two R groups on each of the terminal C atoms are different, are treated as a single sp3 stereocenter.
A compound that has no sp3 stereocenters must be achiral overall. However, it may have chiral conformers, and those chiral conformers may be lowest in energy, and sometimes there may even be a large steric barrier to interconversion of the conformational enantiomers.
A compound that has one sp3 stereocenter must be chiral.
A compound that has more than one sp3 stereocenter may or may not be chiral. If it has a plane of symmetry, it is achiral. If it lacks a plane of symmetry, it is probably chiral.
NOTE: Allenes and iminoketenes, R2C=C=CR2 and R2C=C=NR, in which the two R groups on each of the terminal C atoms are different, are treated as a single sp3 stereocenter.
You might want to look at some definitions of stereoisomers.
Or you might want to look at some examples of the different kinds of stereoisomers.
Or you might find it useful to see a glossary of stereochemical terms.