When I say crash, it simply means that the whole system freezes and does not respond to any keyboard or mouse input. There are many reasons that can cause Ubuntu to crash. It could be due to hardware incompatibility (especially ATI/Nvidia graphics cards), a software conflict or when you run too many memory intensive applications at the same time.
If anyone faces a freeze with Ubuntu where you cannot do anything, then this will certainly be helpful if you want to reboot the OS as cleanly as possible without damaging their HDD's or losing their data.
A full system crash (hang) is very rare on Linux. Here’s the bad news: It is still possible for programs on Linux to misbehave themselves (they are not spoiled, probably just coming down with something, or maybe growing). Perhaps the application went into an endless loop because it was feeling lonely, or just stopped responding because it was pouting when I accidentally mentioned something Microsoft-related.