When the platters are not spinning, the heads rest on the surface of the disk. When the
platters spin up, the heads rub along the surface of the platters until sufficient speed
is gained for them to "lift off" and float on
their cushion of air. When the drive is spun down, the process is repeated in reverse.
In each case, for a period of time the heads make contact with the surface of the
disk--while in motion, in fact.
While the platters and heads are designed with the knowledge in mind that this contact
will occur, it still makes sense to avoid having this happen over an area of disk where
there is data! For this reason, most disks set aside a special track that is designated to
be where the heads will be placed for takeoffs and landings. Appropriately, this area is
called the landing zone, and no data is placed there. The process of moving the
heads to this designated area is called head parking.
Most early hard drives that used stepper motors did not automatically park the heads of
the drive. As a safety precaution, small utilities were written that the user would run
before shutting down the PC. The utility would instruct the disk to move the heads to the
landing zone, and then the PC could be shut off safely. A parameter in the BIOS setup for the hard
disk told the system which track was the landing zone for the particular model of hard
disk. Usually, it was the next consecutive-numbered track above the largest-numbered one
actually used for data.
Modern voice-coil actuated hard disk drives are all auto-parking. On some disks, a weak
spring is attached to the head assembly that tries to pull the heads to the landing zone.
When power is applied the actuator is able to overpower the spring and position the heads
normally. When the power is shut off, the electromagnetic force from the voice coil
abates, and the spring yanks the heads to the landing zone before the platters can spin
down; this can sometimes be heard on older drives as an audible clunk when you
turn the power off. Other disks use a different mechanical or electronic scheme to achieve
the same goal. Some even make use of the rotational energy remaining in the spindle motor to move the heads off the data surface when the power is
cut off! This means that modern hard disks will automatically park their heads--even in
the event of a power failure--and no utilities are required. The BIOS landing zone
parameter for modern drives is ignored.
Some people still think that it is necessary to manually park the heads of modern hard
disks, but this is not true. I sometimes think of head parking utilities as the disk
drive's equivalent of a screen saver. In both
cases, the software was invented as a preventative measure, and one that made sense for
use with the technology that prevailed at the time it was thought up. And in both cases,
the technology has evolved to the point where utility is no longer necessary, yet many
people still think it is. 
IBM has developed an alternative to conventional head parking that I think is really a
great idea. Instead of letting the heads fall down to the surface of the disk when the
disk's motor is stopped, the heads are lifted completely off the surface of the disk while
the drive is still spinning, using a special ramp. Only then are the disks allowed to spin
down. When the power is reapplied to the spindle motor, the process is reversed: the disks
spin up, and once they are going fast enough to let the heads fly without contacting the
disk surface, the heads are moved off the "ramp" and back onto the surface of
the platters. IBM calls this load/unload technology. In theory it should improve
the reliability of the hard disk as a whole, by eliminating most contact between the heads
and platters entirely. I am unaware of any other drive manufacturers using it at this
time. You can read more about it here.
Another feature related to reducing damage to the hard disks caused by wear from the
heads is wear leveling, which moves the heads over the surface of the drive to
avoid "wearing out" one section of the drive. It is discussed in this quality and reliability section.
Next: Single vs. Multiple Actuators
