What is the difference between the original, second generation, and third generation iPod models?

Please note that all iPod models mentioned in this Q&A have been discontinued.

The original iPod, introduced October 23, 2001 and discontinued July 17, 2002, originally shipped with a 5 GB hard drive and shipped with a 5 GB or 10 GB hard drive from March 21, 2002 to July 17, 2002. All configurations have a sleek "iBook white" and stainless steel case with a two inch white backlit LCD display.

Apple continued to sell the original 5 GB "Scroll Wheel" model as part of the second generation iPod series, but assigned new part numbers -- M8513LL/B (for the Mac compatible model) and M8697LL/A (for the new Windows compatible model).

The second generation iPod (Touch Wheel), introduced March 21, 2002 shipped with the 5 GB "hold over" model from the original series, as well as new 10 GB and 20 GB configurations.

The second generation models are similar to the original iPod, but include larger capacity hard drives, replace the "scroll wheel" (that physically turned) with a "touch sensitive scroll wheel" said to be more durable, add a cover to the Firewire port, have a hold switch encased in "clear" plastic rather than white, and include a wired remote control, thinner Firewire cable, and a carrying case. Perhaps most notably, the second generation also introduced models designed for use with Windows.

The third generation iPod, introduced April 28, 2003 and discontinued July 19, 2004, originally shipped with a 10.0 GB, 15.0 GB, or 30.0 GB hard drive, replaced the 15.0 GB hard drive with a 20.0 GB hard drive on September 8, 2003, and replaced the 10.0 GB hard drive with a 15.0 GB drive and the 30.0 GB hard drive with a 40.0 GB hard drive on January 6, 2004.

The third generation iPod has several hardware improvements compared to the earlier models, including a thinner, lighter, and slightly more rounded enclosure, a "dock" for quick-and-easy connection to a computer and audio out for connection to a stereo or powered speakers (models also shipped with a USB 2.0 cable for compatibility with a greater number of PCs).

The now backlit solid-state buttons were moved from around the touch wheel to above the wheel, which some users liked and others disliked, but no one disliked the additional five minutes of skip protection (up to 25 minutes). Software improvements include greater customization options, a voice recorder feature, games, and an alarm clock. Battery life is an estimated eight hours, down from ten hours compared to the earlier models.