This question originally appeared on Quora. 
Answer by Vijay Kamath, coder
Crash is a vague word used to describe multiple reasons of software failure and hardware failures that cause it.
Possible software reasons for a crash
The Operating system provides an interface for users to access the system easily. But is also the source of a lot of software bugs that can cause a computer to crash .
- For instance, when the OS tries to access an incorrect memory address, perhaps as a result of a programming error. In Windows, this can lead to an error known as a General Protection Fault (GPF).
- Software drivers that are added to the OS to allow the use of additional devices (such as printers) may contain bugs and are often the source of crashes.
Possible hardware reasons for a crash
Hardware errors can be devastating, harder to diagnose, and costly to fix.
- Computers have been using hard-disks for a long time. Each disk stores information in units named sectors. More permanent faults happen with errors on a computer's disk. Sectors can go bad during regular use and lose the information stored on them. If these sectors happen to hold system information, they can cause a crash.
- A disk can fail completely when the computer gets jarred and the head that reads information makes contact with the disk surface and scratches it. This may cause all data on the disk to be lost. Some laptops have technology to lift the head when the computer is jarred.
- A computer's power supply converts alternating current to clean direct current. If it starts to fail, the computer can crash accidentally when the power supply generates a noisy signal.
- The random access memory (RAM) can also fail in an intermittent way, particularly if it gets hot. Because the values RAM stores get corrupted unpredictably, it causes random system crashes.
- The central processing unit (CPU) can also be the source of crashes due to excessive heat. The (often loud) fans on most common computers are there to prevent this type of crash, though they may eventually fail.
- The fans that bring cooling air into the case also carry dirt and dust inside. This dirt can accumulate and cause intermittent short circuits as the dirt blows around.
Windows operating system "Blue Screen Of Death" (BSOD)
Blue screens of death are typically caused by software errors in device drivers. They can also be caused by physical hardware faults, such as faulty RAM memory or power supplies, overheating of components, or hardware which is run beyond its specification limits ("overclocking").
Additional reading: Scientific American.