Quit[ ] quits only the Mathematica kernel, not the front end.
To quit a notebook front end, choose the Quit menu item.
All kernel definitions are lost when the kernel session terminates.
If you have kept the definitions in a file or in a notebook you can always re-enter them in a subsequent session.
Before terminating a kernel session, Mathematica executes any delayed value that has been assigned to the global variable $Epilog. Conventionally, this attempts to read in a file end.m of commands to be executed before termination.
On most computer systems, Quit[n] terminates the Mathematica kernel, passing the integer n as an exit code to the operating system.