1. Electron-deficient hydrides

2. Electron-precise hydrides 

3. Electron-rich hydrides
An electron-deficient hydride has very few electrons, less than that required for representing its conventional Lewis structure e.g. diborane ($B_2{H}_6$). In $B_2{H}_6$, there are six bonds in all, out of which only four bonds are regular two centered-two electron bonds. The remaining two bonds are three centered-two electron bonds i.e., two electrons are shared by three atoms. Hence, its conventional Lewis structure cannot be drawn. An electron-precise hydride has a sufficient number of electrons to be represented by its conventional Lewis structure e.g. $C{H}_4$. The Lewis structure can be written as:
                   


Four regular bonds are formed where two electrons are shared by two atoms.
An electron-rich hydride contains excess electrons as lone pairs e.g. $N{H}_3$.
                                      

There are three regular bonds in all with a lone pair of electrons on the nitrogen atom.