Resource loading mainly involves your manpower or employees. In resource loading, each employee is assigned a task or a percentage of a project (X percent of the whole). Usually, it's 25 percent of the whole. Then the employee is assigned other tasks until he or she reaches 100 percent booked. This would then mean that the employees cannot take on any additional work.
With resource loading, a project manager can predict an employee's hours for the year and see how tasks can be assigned. This also allows the project manager to decide whether or not additional employees or contractors are needed to complete the scheduled projects.
The downside to resource loading is that employees cannot be 100 percent booked. Other things may arise to take away their time, such as unexpected problems that need to be fixed. An employee should always be under 100 percent booked. Resource loading increases the chance that a project will not be completed on time because employees are overloaded with projects.
While resource loading mainly deals with manpower, resource leveling deals with both time (project starting and ending date) and resources, including manpower and budget. Resource leveling tries to balance the conflicting interests of projects with the available resources.
Resource leveling generally breaks things down into two categories: time and available resources. Some projects need to be finished within a certain time frame. These projects will use all the available resources (money and manpower) to complete the project by a certain date. For a complete overview of resource leveling, read my article "What is Resource Leveling?"
Projects that aren't as pressing can be spread out for an indefinite period of time until resources do become available. These projects are usually ones that are not on the critical path and will not affect the project completion date.
Like resource loading, resource leveling also has its problems. It is hard to determine in the beginning which tasks will be on the critical path. Also, delaying a task could cause the entire project to fall behind schedule.