Project – It is an effort to come up with a unique solution or product. It is temporary in nature and has a start and endpoint which are defined. The main focus is on detailed delivery.
Program – It is a set of interrelated projects which are managed together. Just like a project, it is also temporary in nature but lasts longer. The program follows high-level plans that are backed up by many detailed plans.
Portfolio – It is a bunch of projects as well as operations that are managed by one group so that the strategic goal can be achieved. Unlike project and program, it is permanent in nature and aligned with the strategic planning.