Critical Tasks are tasks that have a direct impact on the scheduled end date of the project.
Project Insight calculates task relationships within the schedule and marks tasks as critical when there is no "float time" for those tasks. A critical task duration increase of one day will cause the project total duration to increase by one day.
As always, our source for this calculation is the PMBOK (Project Management Body of Knowledge) provided by the Project Management Institute.
It is not uncommon for a project manager see critical task indicators on tasks that are not "critical" in the dictionary sense of the word. Therefore, keep in mind that Critical Path in the project management world is a mathematical calculation. You may be looking for the vocabulary to discuss those tasks that the project manager knows intuitively to be the highest risk activities within the project. In which case a risk register or potential Issue list may provide you the best visibility.