A disproportionation reaction is when an element is both oxidised and reduced in the same reaction. You check this by looking at the oxidation state of the element at the begining of the reaction and then what it is in the two products at the end; if the oxidation state has gone up (i.e it has lost electrons) it has been oxidised, and if the oxidation state has gone down (i.e it has gained electrons) it has been reduced. Remember OILRIG to help;

Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons)

An example is the disproportionation of copper in the following reaction:

Cu₂O _(aq) +  H₂SO_{4 (aq)} --> Cu _(s) + CuSO_4 (aq) + H₂O _(l)

Here the copper goes from oxidation state +1 in Cu₂O to oxidation state 0 in Cu and oxidation state +2 in CuSO_4.