there is no difference

Yes it is. However, some PM Software out there doesn't do the job quite well (of resource leveling), so you're advised to save a copy of your project before applying resource leveling.

Resource leveling - Resource leveling is not an independent schedule network analysis method. It is applied to the schedule that has already been analyzed using other methods, such as the critical path method or the critical chain method. The resource leveling technique is applied to address the resource needs of activities that must be performed to meet specific delivery dates. Resource leveling involves taking a part of the resources from one activity and assigning it to another. This will change the activity durations and can also result in a change of critical paths.

Resource leveling is a schedule network analysis that results in schedule changes, start and finish dates, based on resource constraints. That is, resource leveling might cause start and finish dates to change if multiple activities rely on over-committed resources.

Resource Loading: It is just assigning resources to activities.

Resource Leveling: Any form of network analysis in which scheduling decisions (start and finish dates) are driven by resource management concerns (e.g., limited resource availability or difficult-to-manage changes in resource levels).