Programs vs Projects what is different?

A program is something which delivers a massive piece of functionality (like say Terminal 5 at Heathrow Airport). Within this program will be a large number of individual projects each of which will deliver a specific piece of the functionality. When all these projects have delivered, all their pieces will make the big deliverable. The art of the Program Manager is to make all the individual projects come together at times to suit each other, and also to meet the big deliverable. Imagine how complex this will have been on the Terminal 5 program!