Project management is the application of knowledge, skills, tools and techniques to activities in order to meet the requirements of a project.
The project manager is the person responsible for accomplishing those objectives.
Completing the objectives require the project manager to balance among three costraints: scope, time and cost; the relationship among them is such that if any one of the three factors changes the other are affected as well, i.e. you cannot increase the scope adding more features without increasing also the costs or the time necessary to complete.
Note that many books and experts speak about four factors, adding the quality costraint to the three above but I personally believe that quality should be implicit in project management and you cannot decrease it just to finish earlier.
Some of the skills required are unique to project management, such as work breakdown structures, critical path analysis or earned value management, however effective management requires also knowledge specific to the related area (as standards, regulations and best practices), understanding of the environment (cultural, social, politic) , general management skills (as budgeting, marketing, contracts) and – very important – interpersonal and communication skills.
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