A project is a single temporary endeavor, which creates a new product, service or result. A group of related projects that are centrally managed together and sharing benefits is a program. Finally, portfolios are groups of projects and or programs that deliver maximum value to the business.
The main distinction amongst these three endeavors is that both project and program management focus on the short-term tactical goals whereas portfolio management places emphasis on the long-term or strategic goals.
A good example would be a software company’s strategic goal to increase their customer satisfaction, which would be considered a portfolio. They could then have several projects or programs within such as customer service training. Within this training program would be ever changing projects to better equip the employees who deal with the public more than the others.