Siouxfan asked the Answer Line forum to define a hard drive crash is and explain what can cause it.
When a hard drive spins, the head that reads and writes the magnetic data floats just above the surface of the platter. Originally, the term hard drive crash referred to the head literally crashing into the platter, causing serious damage to your hardware and data.
Today people use the term more generically, referring to any catastrophic incident that renders your hard drive unreadable. Electrical surges, sudden impacts (such as falls or car crashes), and general wear-and-tear can all destroy hard drives.
Remember that a hard drive is a very finely-tuned mechanical device, containing two very different precision motors--one spinning the drive and the other controlling the position of the head. All mechanical devices eventually wear down, and when such a device needs to be precise to extremely small fractions of an inch, it doesn't take much wear to get it out of whack.
When you consider what goes into a hard drive, it's amazing how rarely they crash. I've only had one crashed hard drive, and that was in 1986 (the first hard drive I ever owned, actually; a 20MB--yes, megabyte--Seagate). But the danger is there, and that's just one of the many reasons you need to keep your data backed up.
I'd like to thank rgreen4 for this contribution to the original forum discussion.