No memory system is perfect. Certainly your computer's memory won't mislay its gloves or forget your birthday, but it can suffer little slips, an errant bit here and there. Although one bit in a few million might not seem like a big deal, it's enough to send your system into a tailspin, or worse, alter the answer to some important calculation, jogging the decimal point a few places to the left.
The memory errors that your computer is likely to suffer fall into two broad classes: soft errors and hard errors. Either can leave you staring at an unflinching screen, sometimes but not always emblazoned with a cryptic message that does nothing to help you regain the hours' work irrevocably lost. The difference between them is transience ...