USER :: USER layer is the first layer in UNIX architecture.Here the USERs sends the request to the next layer. USER never directly interacts with the KERNEL.

Request Eg:: Login and Password

SHELL :: It is a Command Line Interpreter(CLI) and it also act as scripting language.Shell converts the high level language to machine level language.
SHELL receives request freom USER and it checks command existence.If command is existed then it converts into KERNEL understandable language(Machine Level).And it passes the given equest to KERNEL.
SHELL acts as interface between USER and KERNEL.

KERNEL:: It is heart of the UNIX.It consists of 100s of system calls.
System call means Low Level Program.All system calls writtern in 'C' Language.
eg., open(),copy(),create(),kill(),proc(),fork()