From Mac Guides
The iMac is Apple's self-contained (aka All-in-One) consumer desktop computer. It was introduced in 1998, and can be considered to be one of Apple's most popular product lines.
The Intel iMac was first introduced on January 10, 2006 in a surprise announcement. The design was initially identical to the final revision of the PowerPC based iMac G5. In August 2007, the design was later revised to a new glass and aluminium enclosure to match the iPhone and to make it appear closer to Apple's professional products.
The iMac G5 was introduced in September 2004 to replace the iMac G4. The new design was, again, a major change and moved all the components behind the screen. The third revision received a minor design change which made it slimmer and rearranged components for more efficient cooling, but the innards were now inaccessible to users unlike the first previous models.
The iMac G4 was introduced in 2002 to replace the iMac G3. It was a radical redesign and featured a highly adjustable flat-panel display mounted on a hemispherical base which contained the computer. Larger 17-inch and 20-inch models were eventually made available in addition to the existing 15-inch model. It was discontinued in 2004.
The iMac G3 was introduced in 1998 and was discontinued in 2003. Its design was unique in that it featured bold colors unlike most other computers of the time.