They specify the number and type of orbital combinations.So, sp3 hybridization is a combination of one s orbital with three p orbitals. sp2 hybridization is a combination of one s orbital with two p orbitals, whist sp hybridization is a combination of one s orbital and one p orbital.They define the bonding properties and shape of molecules. sp3 gives a tetrahedral shape, sp2 a trigonal planar shape, and sp a linear shape.