Polymorphs have the same chemical composition but different crystal structures.
Polymorphisms in molecular crystals arise from packing and/or conformational differences. Conformational polymorphism occurs frequently for flexile molecules which can adopt different conformations in different crystals forms (polymorphs). The energy difference between conformational polymorphs is often small, especially if van der Waals forces dominate the crystal lattice.
Difference in solubility between polymorphs is covered in section Related fields, Solubility.
Polymorphs can be either enantiotropically or monotropically related, see the following section, Thermodynamics of polymorphs.