The term kernel refers to the core of an operating system that supports all other operations. By contrast, the term shell is used to describe the user interface.
Used by a third-party to obtain confidential data (login details, passwords, credit card numbers, PINs, etc.) by intercepting key presses. Backdoor Trojans typically come with a built-in keylogger; and the confidential data is relayed to a remote cybercriminal to be used to make money illegally or gain unauthorized access to a network or other resource.