There are a number of vital factors that influence a project's outcome, which are covered by Project Management theory. The Project Management Institute provides a framework for these factors in its Project Managment Body of Knowledge (PMBOK), a standard adopted by most International Standards organisations (including the IEEE and ISO) as the standard for project management for their respective members.
(An old edition of PMBOK can be downloaded here http://egweb.mines.edu/eggn491/Information
and Resources/pmbok.pdf )
The 9 areas outlined by PMBOK as influencing a project's success, and therefore critical for management, are as follows:
Project Scope, Project Time, Project Cost, Project Quality, Project Human Resources, Project Communications, Project Risk, Project Procurement, and Project Integration.
All of these are the most important considerations, but in particular:
Managing the scope of a project is about ensuring that the work to be done is well defined, so that client expectations can be managed and the project scope will not "creep" - that is, gradually expand, resulting in a project that cannot be achieved with the time, budget, or resourcing originally envisaged.
Managing the timeline of a project is vital, as a project should be completed by the deadline originally envisaged. In many cases, other projects will depend on the succesful and on-time completion of a project; and timely completion of a project is directly linked with the the completion of a project under budget.
Project cost (or budget) is also vital, as funding for any project is finite, and is generally allocated at the commencement of a project. A project's budget be exceeded can result in the incompletion of a project due to an inability to pay for further work. It can also result in other projects not being able to be funded, should their funding be diverted to an over-budget project to keep it running.
Project quality management is about ensuring that the outcomes of a project are of a sufficient standard to meet the client's needs. Without effective quality management, a project may be completed on time and on budget, but be a failure due to the final, delivered product or service not being able to achieve its requirements. Quality management is also about gathering feedback and data throughout and at the end of the project management lifecycle, to continuously improve project management and project outcome quality for future projects.
Project Risk Management is about analysing, prioritising, and addressing risks that may arise during the execution and implementation of a project, to protect project assets, personnel, organisations, and the general public. It is vital for strategically addressing risks to other project areas, as well as for protecting interests outside the project.
Because managing all of these factors is specialised, a qualified and experienced project manager may be required to ensure effective planning, management, leadership and accountability within the project.