The major factor here is the type of the task and effort driven or not. You can make a task fixed duration, fixed work, or fixed units. I will ignore fixed units for this explanation. If a task is fixed duration and effort driven, then no matter how many people you assign to it, the duration won't change. For a fixed duration task however, if it is not effort driven, adding resources increases the amount of work required for the task without changing the duration. If you add resources for a fixed duration task, the utilization rate will decrease for the resources. A fixed work task is the opposite. Adding resources to the task will decrease the duration by spreading the work over the available resources. An example:
Task A requires 40 hours of work to complete and is effort driven with fixed work. If I assign Brian to do task A, it will take him 5 days at 100% utilization. If I want it done sooner, I can add Clark to the task. With Brian and Clark working together, it will only take 2.5 days (20 hours). Brian and Clark will both have 100% utilization.
Task B takes 5 days to complete and is a fixed duration and effort driven. If I assign David to the task with 100% utilization, it will be complete in 5 days and he will do 40 hours of work. If I then add Ethan to help on the task, it will still take 5 days. Now David only has to do 20 hours of work and Ethan does the other 20 hours. So both of their utilization rates are now 50%.
Task C takes 5 days to complete and is fixed duration but not effort driven. If I assign Fred to the task with 100% utilization, it will be complete in 5 days and he will do 40 hours of work. If I then add Greg to help on the task, it will still take 5 days but the work will increase to 80 hours. Both Fred and Greg will still have 100% utilization.
A fixed work task is always effort driven so there cannot be a case where there is fixed work but not effort driven.
You can change these settings by double clicking on a task and going to the "Advanced" tab. Here you can set the Task Type (fixed units, fixed duration, or fixed work) and effort driven or not. Hopefully this helps you understand how the scheduling engine works a little better. Good Luck!
** When a task is fixed duration and effort driven, and you add a resource (as in Task A) the resource utilization rate will not appear as a percentage in the resource column next to their name. It will show both of them as a regular resource with 100% utilization but they will only be working 20 hours each, so if you assign them to a second concurrent task with the same type, duration and work, there will not be an overallocation because they can work up to 40 hours. You can see how many hours of work a resource has by going to the resource sheet and viewing the "Regular Work" column.