Storage

• The first generation of iPods could hold only 5 GB worth of data. The storage capacity on the classic iPod models has steadily increased over time: Second-generation "Clck Wheels" were available with up to 20 GB capacity; fourth-generation iPod Photo could hold up to 60 GB; and the iPod Classic is available with up to 160GB of data. Smaller iPods, both in physical size and capacity, have also been introduced. The 2004 iPod mini could hold 4 GB of music data.
  
  

Design

• The iPod Classic retains the basic structure of the original iPod, coupling a display with a circular control wheel on a rectangular body. The iPod Mini, released in 2004, and the first- and second- generation iPod Nano devices, released in 2005 and 2006, respectively, had the same basic design in a smaller scale. The iPod Touch, introduced in 2007, was very similar to the iPhone design, eliminating the Click Wheel and instead extending the display. In contrast, the original iPod Shuffle, released in 2005, removed the display element completely. Subsequent iPod Shuffle designs have continued to make the overall device even smaller.
  
  

Display

• The first generations of iPods, including the Scroll Wheel, the first Click Wheel, and both generations of the iPod Mini, featured a monochromatic display. In October 2004, Apple debuted the iPod Photo, which was also known as the iPod with the color display. The iPod Nano replaced the iPod Mini in September 2005 and also featured a color display. The iPod Touch debuted in 2007, with a larger display that included a touchscreen interface.
  
  

Controls

• The first-generation iPod is called the "Scroll Wheel" iPod. It featured a variety of button-based controls and a physical scrolling wheel that allowed users to scroll through the iPod's menus. The iPod "Touch Wheel" came next, and looked very similar to the scroll wheel, but the wheel mechanism was touch-sensitive and did not turn. The Click Wheel was introduced with the fourth generation of iPods. It retained the general elements of its click-wheel predecessor, but had no moving parts of its own.