The original iPod (mp3 mp4 player) introduced on 23 October 2001 marked the first iPod generation, and the latest iPod introduced on 12 October 2005 marked the fifth iPod generation, which was subsequently revised with a revision iPod model on September 2006, often referred to as generation 5.5. Note that the generations are referred with reference to the model name and not with reference to the date of release.
For a better understanding about iPod generations, check the following:
On October 23, 2001, the first generation of iPod was released. The model name was iPod and had the storage capacity of 5 to 10 gb. It used FireWire connection to computers and it had a mechanical scroll wheel.
On July 17, 2002, the second generation of iPod was released. It had the storage capacity of 10, 20 gb. It used FireWire connection to computers and it had a touch-sensitive wheel. The Hold switch was revised. The FireWire port had a cover.
On April 28, 2003, the third generation of iPod was released. It had the storage capacity of 10, 15, 20, 30, 40 gb. It used FireWire connection to computers (USB for syncing only), and it had a central row of touch sensitive buttons. New backlit screen was introduced. Dock connector port was introduced.
On January 6, 2004, the first generation of iPod mini was released. It had the storage capacity of 4 gb. It used FireWire or USB connection to computers, and it introduced the “Click Wheel”, which was later adopted by the fourth generation iPod. It was available in 5 colors.
On July 19, 2004, the fourth generation of iPod was released. It had the storage capacity of 20, 30, 40, 60 gb. It used FireWire or USB connection to computers, and buttons were integrated to form “Click Wheel”. The monochrome model was replaced in June 2005 by the color display with photo viewer, which was introduced in October 2004.
On January 11, 2005, the first generation of iPod shuffle was released. It had the storage capacity of 512 mb, 1gb. It used USB connection to computers, and it had no screen, no click wheel. It used flash memory instead of hard drive storage.
On February 22, 2005, the second generation of iPod mini was released. It had the storage capacity of 4, 6 gb. It used FireWire or USB connection to computers , and it had longer battery life, and brighter color variants. Gold model discontinued. No AC adapter. The body color matched the click wheel lettering.
On September 7, 2005, the first generation of iPod nano was released. It had the storage capacity of 1, 2, 4 gb. It used USB connection to computers (FireWire for charging only), and it was the successor of iPod mini. Slimmer design, flash memory, color screen and lyrics support.
On October 12, 2005, the fifth generation of iPod was released. It had the storage capacity of 30, 60, 80 gb. It used USB connection to computers (FireWire for charging only), and it had large screen with video player and lyrics support. No AC adapter, Universal Dock, or A/V cables were included. It had black and white color variants. This model was revised in September 2006 (often called as generation 5.5) had a brighter display, a music search function, and a longer video battery life.
On September 12, 2006, the second generation of iPod shuffle was released. It had the storage capacity of 1 gb. It used USB (via dock only), and it had the aluminum case with smaller form factor. Built-in clip. Later multi-colored models were also released.
On September 12, 2006, the second generation of iPod nano was released. It had the storage capacity of 2, 4, 8 gb It used USB connection to computers (FireWire for charging only), and it had the anodized Aluminum case in 6 colors. Music search function, longer battery life, and brighter screen.
When more refinements are carried out with respect to features, size, weight and etc., a new generation is announced of the respective model. The distinguishing landmarks were the replacement of the mechanical scroll wheel by touch-sensitive click wheel, the replacement of the hard disk by the flash memory, and the replacement of the monochrome display with color displays. Earlier generations are usually discontinued in favor of newer generations. Earlier generations had bundled software compatible with Macintosh only, while later iPod (mp3 mp4 player) generations have both Macintosh and Windows compatible versions.