Project stakeholders are the people who are involved in or somehow affected by the project. Project sponsor, project team, upper management, users, customers - are all examples of project stakeholders. The key project stakeholders are the stakeholders who have serious power and abilities to influence the project's outcome. Key stakeholders have to be identified for each project individually, but most obvious key stakeholders are senior management and project sponsor.
Key stakeholders such as senior management, using their high position in the company, can help the success of the project by speeding up the execution of some tasks performed by other departments such as paperwork by legal department, financial documents by finance department, and etc.
Not only the key stakeholders, but the users of the system, can help the success of the project by cooperating in requirement gathering and then during the testing of the new system.