It is always preferred to shut a Mac down or restart it by using the options in the Apple menu. This allows services to be stopped, settings and documents to be saved, and hardware to be cleanly unmounted and put in a state where it's ready to be shut off. However, there are times when a hard reset may be necessary, which besides obvious kernel panics, includes system freezes and hangs. These are generally characterized by the following behaviors:
Kernel panics: When these happen, the screen darkens and instructs you to press and hold the power button.
System freezes: … Read more