Safe mode is a diagnostic mode of a computer operating system (OS). It can also refer to a mode of operation by application software. Safe mode is intended to fix most, if not all problems within an operating system. It is also widely used for removing rogue security software.
Operating system safe mode 
Microsoft Windows, Mac OS X and Linux distributions such as Ubuntu and Linux Mint are examples of contemporary operating systems that implement a safe mode (called "Safe Boot" in Mac OS X); as well as other complex electronic devices.
An operating system in safe mode will have reduced functionality, but the task of isolating problems is easier because many non-core components are disabled. An installation that will only boot into its safe mode typically has a major problem, such as disk corruption or the installation of poorly configured software that prevents the operating system from successfully booting into its normal operating mode.
Though it varies by operating system, typically safe mode loads as few executable modules as possible and usually disables devices, except for the minimum necessary to display information and accept input. Safe mode can also take the form of a parallel "miniature" operating system that has no configuration information shared with the normal operating system. For example, on Microsoft Windows, the user can also choose to boot to the Recovery Console, a small text-based troubleshooting mode kept separate from the main operating system (and can also be accessed by booting the install CD), or to various "safe mode" options that run the dysfunctional OS, but with features such as video drivers, audio and networking disabled.
Safe mode typically provides access to utility and diagnostic programs so a user can troubleshoot what is preventing the operating system from working normally. Safe mode is intended for maintenance, not functionality, and provides minimal access to features.
Microsoft Windows' safe mode is accessed by pressing the F8 key as the operating system boots. Also, in a multi-boot environment with multiple versions of Windows installed side by side, the F8 key can be pressed at the OS selector prompt to get to safe mode.
An equivalently minimal setting in Unix-like operating systems is single-user mode, in which daemons and the X Window System are not started, and only the root user can log in and which allows an administrative account commonly known as "root" to have its password reset without knowing what it is first. On Mac OS versions 6, 7, 8 and 9, a similar mode is achieved by holding down the shift key while booting, which starts the system without extensions. In Mac OS X holding the shift key after powering up activates Safe Boot that has background maintenance features (Besides the mode selection, it runs a file system repair, and in Mac OS 10.4, it disables all fonts other than those in /System/Library/Fonts, moves to the Trash all font caches normally stored in /Library/Caches/com.apple.ATS/(uid)/, where (uid) is a user ID number such as 501, and disables all startup items and any Login Items) and can reset an administrative accounts' password. In Windows, safe mode with networking, one of the variations of safe mode, can be used to troubleshoot network issues. In OS X, Safe Boot always includes networking.
Application software safe mode 
Application software sometimes offers a safe mode as well. In the PHP interpreter, prior to version 5.4, safe mode offers stricter security measures. The Glasgow Haskell Compiler from version 7.2 offers "Safe Haskell" mode, restricting usage of functions such as unsafePerformIO. Mozilla Firefox's safe mode allows the user to remove extensions which may be preventing the browser from loading. Internet Explorer can run in "No Add-Ons" mode and Protected Mode.