Parallel or audit testing is a type of testing where the tester reconciles the Output of the new system(new version) to the output of the current system, in order to verify the new system operates correctly.

Example:Operating new and old versions of a payroll system to determine that the pay checks from both the  systems are reconcilable

parellel testing is process of testing.
During this testing one s/w compare with previous version 
of same s/w, to estimate strengts and weakness of that s/w.