Apple introduced the world to the iPod on October 23, 2001. Seven years later, that US$400 music player has grown into a full lineup of music and video devices that dominate the portable digital media player market and set the bar for all other media players.

Since its inception, Apple's iPod has transitioned from a hard drive-based Mac only device to a cross-platform media player, added support for video playback, expanded to include flash memory-based models, and added models with touch-based interfaces.

Apple's original iPod

The first iPod cost $399 and measured 2.43 x 4.02 x .78-inches. In comparison, the 120GB iPod classic -- the model that most closely physically matches the original -- costs $249, and measures 2.4 x 4.1 x .41-inches. Where the original was essentially just a music player, later models like the iPod classic can also play videos and even games.

In January 2004, Apple introduced a smaller form factor model called the iPod mini. The smaller sized unit shipped with a 4GB hard drive and was available in silver, pink, gold, blue, or green. The popular iPod mini was replaced in September 2005 with the iPod nano.

Analysts and Apple fans questioned the company's decision to kill the wildly popular mini and replace it with the flash-based nano. In the end, however, it appears that Apple made the right move because the nano went on to become the best selling iPod model.

While the original iPod nano shucked the metal body found on the mini, later models returned to the more durable shell. That metal shell design carried through a form factor change that saw the nano go short and fat and back to it's long and tall format with the recent release of the fourth generation model.

Apple's 4th generation iPod nano.

Where the plastic-clad nanos were available only in black or white, the metal clad versions have been available in several colors.

The iPod lineup was rounded out in January 2005 when Apple introduced the gum stick-shaped iPod shuffle during Macworld Expo in San Francisco. It was later replaced with the an even smaller second generation model in September 2006. Both were flash-based, like the iPod nano, but did not include a display. The shuffle's compact size and low price helped it to become surprisingly popular as well.

Apple introduced a special edition iPod nano in October 2006 as part of the (PRODUCT) RED promotion to raise money for AIDS programs in Africa. The special edition models carried over into the third generation iPod nano and iPod shuffle, too.

In June 2007, Apple turned the iPod on its ear, or at least its side, with the release of the highly anticipated iPhone. The iPhone blended a video capable iPod, Wi-Fi based Web browsing system, and a cell phone into a single unit. It included a multi-touch interface, automatically rotated its display when held upright or pivoted on its side, and became a hit with consumers as soon as it was released.

Despite Apple's exclusive distribution deal with AT&T in the United States, over a million iPhones were sold by the end of September.

When the iPhone 3G hit the streets with faster data service and a built-in GPS in July 2008, shoppers were able to pick one up in about 20 different countries, and Apple has been expanding into other markets ever since. The company hopes to be selling the iPhone 3G in some 70 countries by the end of 2008.

The original iPhone was followed only a couple of months later by the iPod touch: an iPhone-like iPod that lacked Bluetooth and cell phone features. A second generation version was released in September 2008 that sports an even thinner body and a built-in speaker.

The 2nd generation iPod touch.

Even though Apple's iPod road seems to be paved with gold, there have been a few potholes along the way. The iPod has been plagued with a series of lawsuits over batteries, scratched screens, product pricing, and storage capacity. Apple has been able to resolve most of the suits brought against it, and has been improving its players along the way, too.

The iPod's popularity also helped propel the iTunes Store, originally the iTunes Music Store, into the single largest legitimate music download site. When the iTunes Store first launched it only offered music, but later added music videos, movies, and TV shows. Where other music download services offered music subscriptions, Apple offered music sales, a feature that ultimately has proven to be far more popular among consumers.

The iPod skyrocketed in only a few short years from being a "why would anyone pay that much for a music player?" product to a market dominating must-have device. Apple has been able to maintain that 70 percent-plus market control for the past few years by continuing to update and innovate, and by maintaining an easy to use and slick looking product lineup.

Apple's iPod lineup -- which currently includes the iPod classic, iPod nano, iPod shuffle, and iPod touch -- has been able to stay on top of the portable media player market for years. Its competitors still haven't been able to duplicate the iPod's magic, so for now it looks like the iPod will keep going strong.