Both are schedule network analysis methods.

When you are preparing a schedule for the project you will try to get the critical path for the project. After taking into account dependencies / constraints you would have prepared the schedule. Now you will have to check the resource situation. If you dont have enough resources then you will have to alter the schedule. This resource constrained critical path is called critical chain.

Now resource levelling does not always result in extension of critical path. In resource levelling you are checking if all resources are loaded around 100%. If someone is above 100%, then you will have to either extend the schedule or move some tasks to other resources.

Another important thing that you do in critical chain is add buffers to manage uncertainties. Since you are resource constrained , chances are there will be some unplanned leaves or unavailabilities of resource. In order to account for that feeder buffers and project buffers are added in critical chain method.