A single hard disk usually consists of several platters. Each platter requires two read/write heads, one for each side. All the read/write heads are attached to a single access arm so that they cannot move independently. Each platter has the same number of tracks, and a track location that cuts across all platters is called a cylinder. For example, a typical 84 megabyte hard disk for a PC might have two platters (four sides) and 1,053 cylinders.

In general, hard disks are less portable than floppies, although it is possible to buy removable hard disks.

Also see What's Inside a Hard Drive? in the Did You Know . . . ? section of Webopedia.

Hard Disk Logical Structures and File Systems 
Describes "standard" file systems used on PCs, with particular focus on the FAT file system and its variants. This page is from "The PC Guide."

PC Guide's hard disk drive reference 
Offers a large amount of information covering construction and operation, geometry and formatting, performance factors, BIOS and size issues, interfaces, configuration and file systems.

Disk Manager product page
Describes Ontrack Computer's Disk Manager product, a hard disk installation package.

Hard Disk BIOS and Capacity Factors
Describes issues related to how the BIOS and operating system interact with the hard disk, and BIOS-related issues and problems. Includes a full look at the many capacity limitations inherent in using IDE/ATA interface drives, and other BIOS restrictions on hard disk capacity. This page is from "The PC Guide."

Hard Disk BIOS Translation Modes
Detailed description of the various types of BIOS translation that are used in modern PCs. Includes sections on normal/standard CHS mode, Extended CHS/large mode, Logical Block Addressing (LBA), a comparison of translation modes, and caveats on changing translation modes and transferring hard disks between PCs. This page is from "The PC Guide."

Hard disk information page
This site hosts a collection of information on hard disks, including an EIDE FAQ, the bus master DMA features and drivers and troubleshooting, driver information, and manufacturer pages.