hard disk drive
An HDD in a nonremovable form of computer data storage. Measured in bits (and subsequently bytes, kilobytes, etc.), it uses magnetism and the binary system to store most of the things that makes your computer work, and your personal files.
It consists of at least one platter, which is a single disk that is rotated around a spindle and read by a head. The speed that the platter can be accessed with is measured in RPM, though a higher RPM is needed as the capacity of the HDD becomes greater.
After a set period of inactivity, the HDD spins down, which means the platters stop rotating to save energy. The drawback is the neccesity to spin back up when you use the HDD again, which takes a few seconds. When the HDD spins down, the head, which is attached to an arm moves from its reading position from over the platter to its resting position which is not over the platter. This is much safer for your HDD, because when you move your computer, or the HDD itself, the arm will not be shaken and damage part of the platter.
Most of the HDD is stored within an air-tight area. This is to prevent dust and any other small particles from entering the main area and ruining the (very sensitive) platters. If the HDD is cracked open, or forcibly opened, it is pretty much guaranteed that the HDD will not work next time that it is used.
Each HDD has a spot which is used to plug a power cable from the power supply, a plug for the ribbon cable (usually EIDE or SCSI) which connects it to the BIOS, and a series of pins that are jumper settings. You can use the jumper settings to set your HDD to either master or slave. This designates its priority on the daisy chain of ribbon cable. Master will give it priority over slave, and this is the preferred setting, usually leaving a CD-ROM drive to be slave.
When first coming out for in-the-home use, HDDs generally had just a few MBs, but have grown amazingly fast (thanks Moore) to now be 80GBs. Thought to be very large now, 80GBs will most likely be small in just a few years. The average price of the newest HDDs has been around $200 for a while now, and will most likely stay that way (at least until spintronics becomes the norm).