... and Project Management

Programs usually combine projects of a similar kind. The different to projects is the link to business strategy and benefits, and that programs are responsible for multiple deliverables instead of a product. On the other hand, the definition of a "product" can depend on ones perspective. Building the International Space Station may be a product or deliverable, and a system itself, but each sub-product or sub-system can and is defined as one as well. 

Therefore, programs are but natural extensions onto another layer of complex systems of projects. 

... and Complexity

From the perspective of project management, all points applicable for projects are also valid for programs. They are simple extensions of similar projects, adding another layer to their complexity. Programs can be interpreted as higher-ranking, and thus more connected, nodes in business systems that consist on their lowest level of activities performed by individual persons or machines.