The shell is used to allow the user to interface with the operating system and run their applications. BASH is a shell, as is GNOME (Often referred to as a "graphical shell.").

The kernel, on the other hand, is the core of the operating system. It manages the hardware, resources, and processes running on the sysem.

There are actually four major architectural components of Microsoft Windows. These are Windows kernel, core processes, system startup, and the graphical subsystem.

The Kernel

Windows is a graphical environment operating system.

Yes, they are the interface to kernel functionality. To quote wikipedia: a system call is how a program requests a service from an operating system's kernel that it does not normally have permission to run