MSConfig, also known as System Configuration or Microsoft System Configuration Utility is a system utility to troubleshoot the Microsoft Windows startup process. With the exception of Windows 2000 it is bundled with all Windows versions since Windows 98.
Its functionality varies depending on the windows version. Generally, it can disable or re-enable software, device drivers and Windows services that run at startup, or change boot parameters. Starting with Windows Vista it also provides a convenient list of diagnostic tools and other advanced tools the user can run.
Microsoft has published an article on Using System Configuration that covers all of the basics. This article relates to Windows Vista and later.