Microsoft Windows popularised the concept of booting to safe mode, which in Windows' case is a stripped-down version of the operating system where only the essentials are loaded upon booting.
Although it's not used anywhere near as much, OS X also has a safe mode, and it does - or should we say, doesn't do - the following:
Only essential kernel extensions are loaded (a.k.a. ketxs, or hardware and software drivers)
Startup apps and login apps/services are not loaded
Fonts you've manually installed are not loaded
Additionally, system and font caches are automatically cleaned, and as part of the boot procedure the hard disk is verified and attempts made to repair issues with directories -- a little like Windows' FDISK command-line app, although what happens is identical to what would happen if you click the Repair Disk button found in of OS X's Disk Utility.
Booting into safe mode on Mac OS X
Sound useful? All you need do to boot to OS X safe mode is hold down the Shift key just before the Apple logo appears when you reboot or start-up your Mac. When the progress bar appears you can lift your finger. Unlike with Windows there's no text on the desktop once Safe Mode starts to show you're booted into it, and you won't be limited to a tell-tale low screen resolution either. However, clues will be present in the fact the system might seem slow to respond, and animations might appear jerky.
You can boot into safe mode on a Mac by holding down Shift just before the Apple logo appears, and then watching the progress display
So, what can you do in safe mode? Not much! Aside from the repairs mentioned above, safe mode is designed to let you test your Mac. If a problem you've been having doesn't occur when you boot to safe mode then it's a safe bet it's related to a problematic kernel extension (perhaps faulty hardware that kernel extension accesses), or - and this is more likely - it's related to a third-party app or service configured to start with OS X.
To prune your startup app list, open System Preferences, click the Users & Groups icon, select your username at the left, and click the Login Items tab. Select an item then click the minus button beneath to remove it. Some apps and services hide away in system folders, however, and pruning them is only for advanced users. Removing kernel modules is again only for experts, although on modern releases of OS X it's pretty hard for developers and hardware vendors to install third-party modules thanks to the requirement for them to be digitally signed, so this is much less likely to be the cause of any issues.
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