Since something has last been written here, hard disk capacity has, as expected, increased, although much as with processor speeds, the increase seems to have leveled off a little. What hasn't changed, which is also what wasn't mentioned above, and what I am going to write about, is a few of the basic physical facts about hard disks (or hardrives, as others, including myself, would prefer).
For all the changes in computers in the past 20 years or so, some things have stayed relatively the same. One of these is the physical appearance of the hard disk, with a 250 Megabyte hard disk from 1994 and a 250 Gigabyte hard disk from 2006 being virtually identical, if the labeling were to be removed. Almost all IDE hard disks are 3.5 inches wide, by 5 inches long, and weigh about one pound. They are connected to the motherboard of the computer by a cable to carry information, and are connected to the computer's power supply by cables to carry electricity (as opposed to components that use a single connection for both power and data). They also have some pins for jumpers so that the identity of the hard disk as master or slave can be set. (That particular question would better fit under IDE) The hard disk will also have a label, identifying its manufacturer (there are only a half-dozen companies that make hard disks), and also, if you can decipher it, the number of platters, the date of manufacture, and the capacity (which is measured in "decimal gigabytes", not binary gigabytes-another topic best discussed elsewhere). The hard disk will usually have a small controller board attached to its underside, which does all the thinking for the hard disk. The hard disk will also have a number of holes for screws, where the disk is usually connected to the rails or cages of the computer. As noted, except for the date of manufacture and the labeling of capacity, there is no way to know what the hard disk can do from the outside.
There are of course, many exceptions to the general pictures of hard disks. First off, there are early IDE drives that have a much larger form factor. These are museum pieces, however, and shouldn't be worried about. There was also, well into the 1990s, a 5.25 inch wide form factor hard disk, included in many Compaq computers. These were not popular, as they were hard to interchange into other computers, but they are out there. There is also various sizes of SCSI disk, which will often have a different size, and a few different features as far as data connection goes. There are laptop hard disks, which mostly use a 2.5 inch wide form factor, and are of course much smaller and lighter. There is also the new SATA drives, which while roughly the same size as a common IDE disk, have different connectors. And finally, there are always things that are hard to categorize: there are many exotic architectures in computing, and somewhere someone has a hard disk that consists of a bunch of DNA in a tank of nutrient fluid. However, on the whole, hard disks come in a limited amount of shapes and sizes, and change those shapes and sizes slowly over time.
The reason I went to some length to describe this is it often surprises me how little people, even computer people, know about the inside of their computers. Hard disks, like the rest of the inside of the computer, are not just amorphous blobs, but are actual physical objects. A little familiarization with them helps people demystify the inner workings of computers. Also, armed with this knowledge, you can answer the annoying question "How big is your hard drive" with the correct answer: "about 16 ounces".