Cloud:

Everything works locally, unless you use cloud-specific functions (e.g. CloudEvaluate). Curated data (functions like GraphData) and the Entity stuff is downloaded from the cloud, and need an internet connection. This is just data though, it's not computation.

Benchmark:

The benchmark does not use clusters automatically.

Clusters:

This helps only when you use the parallel tools (functions like ParallelTable). http://reference.wolfram.com/language/ParallelTools/tutorial/Overview.html

You would also need to set everything up first, and unfortunately not documented in great details. See here: http://reference.wolfram.com/language/ParallelTools/tutorial/ConnectionMethods.html I guess you would need to define the remote kernels using RemoteMachine, and add them to $ConfiguredKernels before you can just use LaunckKernels and have everything work.

You would need to set up a way to launch executables seamlessly on all the nodes. One way is to set up public key SSH authentication, so you can run remote commands without entering a password as ssh machineName command. Then adjust the templates inside RemoteMachine as necessary.

Expect to spend a few hours before you manage to get all this working smoothly for the first time.