Safe Mode (SM) is a cut-down version of Windows with minimal support that provides just enough software and device-driver support to get a PC running adequately, making it almost impossible for third-party software, including malware and viruses to run. Hence its name of Safe Mode. Malware scanners and other tools, such as disk defragmenters, are designed to work in SM. Using a malware scanner in SM is more effective than using it in normal mode because it has a clear run of the system.
Booting into SM is necessary if you want to remove bad software or get rid of a malware infection, etc., that is or isn't preventing Windows from booting into normal mode.
The version provided by Windows 8.0/8.1 has not changed much from how it is in earlier versions of Windows. The most noticeable difference is the increased screen resolution from 800x600 pixels to 1024x768, which allows more images and information to be shown on the screen.
If you are using Windows 8.0, you should upgrade to Windows 8.1. There is plenty of information on the web on how to do that. If you want to take advantage of the free upgrade to Windows 10, released on July 29, 2015, only Windows 7 SP1 and Windows 8.1 qualify for it, not Windows 8.0. Here is the Microsoft Support Lifecycle for Windows 8.0. You can find the same information about Windows 8.1 from that page.
If Windows encounters a problem that makes it impossible to boot in normal mode, it will attempt to boot into Safe Mode or provide the boot menu that allows the user to boot into SM. It is possible to enter SM in Windows 8.0/8.1 manually, both from a cold boot and from within Windows, but pressing the F8 or Shift + F8 at startup does not work if the computer uses a SSD drive to boot the system or a UEFI BIOS, because the system boots far too quickly. The computer must be using a standard hard disk drive and a standard BIOS.
Note that booting from a System Recovery CD/DVD that can be created by just typing system recovery disk and following through, only works in Windows 8.0. Windows 8.1 can only boot from a System Recovery memory stick/flash drive, which is also created from within Windows 8.1 in the same way as in Windows 8.0.
To boot into Safe Mode from within Win8/8.1, press the Windows key plus the R key to bring up the Run box and enter msconfig in it. A window comes up with several tabs. Open the Boot tab and enable the Safe boot option and then enable the type of Safe Mode that you want to boot into. The options are as follows:
Minimal and Network are the options that most users want to use. Then you just have to reboot and the computer will start up in the Safe Mode that you chose.
The option Network (Safe Mode with Networking) on the boot menu provides access to the PC's network, which it has if it is attached to a router by cable or is wirelessly connected to broadband or dial-up telephone or cable connection. If that is the case, the user can use that option to get online.
Remember to disable the Safe boot option before you leave Safe Mode or from within Windows or the computer will keep booting into it.
These instructions can be printed if you feel that you can't memorise them.
Here is what Help & Support in Windows 8 - found by bringing out the Charms bar on the Start screen and searching for it under Search => Apps - has to say about an alternative and more long-winded method of booting into Safe Mode from within Win8:
Should you need them or just want to know of them, the following page provides alternative ways of entering Safe Mode in Windows 8.0/8.1.
5 Ways To Boot Into Safe Mode In Windows 8.1 -