Portfolio Management = Optimizing the returns and strategic alignment of your project/program investments.
Program Management = Gaining benefits from the management of integrated projects as a whole which couldn't be achieved through managing them individually.
Project Management = Increasing the level of predictability regarding project outcomes.
You can start with a very brief description of what is a project, portfolio, and program in terms of your own products. Then I would list individual characteristics and their benefits to you. These are quick statements: A characteristic of portfolio management is that you are trying to cover a defined market space. A benefit is that you ensure you cover everything while limiting duplicate effort across project teams.