Not always a project will run alone.
Project Portfolio Management (PPM)Edit
A portfolio is composed of programs or projects without dependency between them. It aims strategic business objectives. Portfolio management is responsible for prioritization of program/projects and one of its goals is to have a better resource usage among them. A portfolio manager is responsible to manage several portfolios.
One example or portfolio would be to improve facility services. It would be composed of a program to improve water, other to improve electricity, other to improve gas.
A program is a collection of related projects, all of them with similar goals. If projects have similar goals they would have a better performance through a centralized management. They will have also an improved resource utilization in those cases.
A project management is application of tools an techniques to achieve a specific goal, reaching committed deadline and budget.