What is Safe Mode?:
Safe Mode is a diagnostic startup mode in Windows operating systems.
What is Safe Mode Used For?:
Safe Mode is used as a way to gain limited access to Windows when the operating system won't start normally.
How To Access Safe Mode:
See How To Start Windows in Safe Mode for tutorials for your version of Windows.
If you are able to start Windows normally, but would like to start in Safe Mode for some reason, one really easy way is to make changes in System Configuration. See How To Start Windows in Safe Mode Using System Configuration for instructions on doing that.
If neither of the Safe Mode access methods mentioned above work, see How To Force Windows To Restart in Safe Mode for instructions on doing just that, even if you have zero access to Windows right now.
How To Use Safe Mode:
For the most part, Safe Mode is used just like you use Windows normally. The only exception to using Windows in Safe Mode as you would otherwise is that certain parts of Windows may not function or may not work as quickly as you're used to.
For example, if you start Windows in Safe Mode and want to roll back a driver, you'd do that just as you would do when using Windows normally.
Safe Mode Options:
There are actually three different Safe Mode options available. Deciding which Safe Mode option to use depends on the problem you're having.
Here are descriptions of all three and when to use which:
Choose Safe Mode if you can't access Windows normally and you do not expect to need access to the Internet or your local network.
Safe Mode with Networking
Safe Mode with Networking starts Windows with the same set of drivers and services as Safe Mode but also includes those necessary for the networking services to function.
Choose Safe Mode with Networking for the same reasons you'd chose Safe Mode but when you do expect to need access to your network or the Internet.
This Safe Mode option is often used when Windows won't start and you suspect you'll need access to the Internet to download drivers, follow a troubleshooting guide, etc.
Safe Mode with Command Prompt
Safe Mode with Command Prompt is identical to Safe Mode except that Command Prompt is loaded as the default user interface instead of Explorer.
Choose Safe Mode with Command Prompt if you've tried Safe Mode but the taskbar, Start screen, or Desktop doesn't load properly.
Safe Mode Availability:
While in Safe Mode, the Desktop background is replaced with a solid black color with the words Safe Mode at all four corners. The top of the screen also shows the current Windows build and service pack level.