Crucial tasks are those activities that have the most potential to affect either the overall duration or cost the project. It is in fact of measure of the overall sensitivity of a specific project parameter (cost or duration) to a particular activity. Crucial tasks on the critical path, but not always. A key difference is that CP only takes into account schedule structure, but does not account for risks and uncertainties which can affect the duration of an activity.
A more full explanation can be found here:
http://www.intaver.com/Help3/RP3_HelpPr ... Chart.html
Hope that helps.