What is the difference between File | Exit option and the File | Fast Exit option?
The File | Fast Exit option is an alternative option to close Toad, where it will automatically save any changes to your saved Workspaces and also make any commits needed for open connections, but it will not prompt you to save any files. For example if you have open Editor windows with statements (or anything) typed in the Editor, it will just close them down.
The normal File | Exit will prompt for all these things (the same as if you press the 'X' button in the top right corner to close Toad). The Fast Exit option is just a quicker way of closing Toad, and can be helpful if you have many windows open for example but don't need to save any files.