From Jaguar (OS Version 10.2.x) onwards apple has offered a safe boot option.If your mac is freezing,not starting correctly,any hardware failure,software corruption,then you need to login in to safemode to troubleshoot the issues.
To start a mac in safemode,please follow these steps.
Shutdown your Mac
Press and hold the shift key and turn the computer on.
Release the shift key when you see the login window.
Please note that some features won’t be avaialble in the safe mode.For example DVD player won’t work,imovie won’t allow you to capture videos,Audio iN or out won’t worknetwork file share will be disable.airport cards won’t work.
Safe mode will check the following things
This will move all font caches to the trash.perform a directory checkup of your startup,load only the minimum kernal extensions OS X needs to run.delete the dynamic loader cache,disable all startup items.