What is the difference between user and kernel modes in operating systems?

Question

What are the differences between User Mode and Kernel Mode, why and how do you activate either of them, and what are their use cases?

Answer 1

I'm going to take a stab in the dark and guess you're talking about Windows. In a nutshell, kernel mode has full access to hardware, but user mode doesn't. For instance, many if not most device drivers are written in kernel mode because they need to control finer details of their hardware.

See also this wikibook.