Amphoteric compounds

 Amphoteric compounds are those which behave like acid as well as base.

Best example is water 

H2O + NH3------------> NH4+ + OH-

In this reaction water act as a acid because it donates proton to the base.

H2O + HCl------------> H3O+  + Cl-

In this reaction water is a base because it accepts proton from acid.