Integration testing is the next level up. It is concerned with confirming that no problems arise out of combining unit components into more complex processes. For example, two discrete functions might appear to test successfully in isolation, but if function B is fed the output of function A as one of its parameters, it might not perform as expected. A possible cause might be incorrect or insufficient data validation. Using the previous example of the calculation of the single net salary figure, the actual system might implement a menu or command button option to calculate the salaries for all employees and produce a report of the results. It is this entire routine that would qualify as an integration test.
As with unit testing, it is important to write test plans that will execute along all conceivable paths between the units. Integration testing, by its nature, will probably be performed by a dedicated tester except for small projects.