Effort-driven scheduling means that the total work for a task drives the calculations of duration, units, and work for individual assignments. If you add or subtract resources, the total work remains the same, while Office Project 2007 adjusts the task duration or units at which resources are assigned.
On the other hand, if you disable effort-driven scheduling, the total amount of work will change if you add or remove resources, just as the number of person-hours increases as you ask more people to attend a two-hour meeting. Essentially, effort-driven scheduling works hand-in-hand with the task type.