AMD
Introduction
AMD is a chip making company that creates the Radeon line of GPUs, in addition to several lines of CPUs.
Radeon
The Radeon line of graphics cards were introduced in 2000 with the Radeon R100.
Radeon HD Series
The Radeon HD series began in 2006 with the R600 chip series. It was the first line of Radeon GPUs to support DirectX 10 and beyond. The HD marked a change in the Radeon naming scheme, one in which the chip series could be easily discerned by the name. The most recent entries in this series are the HD 7000 line.
As an example we can use the Radeon 5670.
Here the first digit signifies the series, in this case the 5000 series. The second digit signifies the power range, with a mid-range card signified by the "6". The third digit indicates a further ranking, with the 5670 being slightly more powerful than the 5650. The final digit is always 0.
With the release of the 12.6 drivers, only the Radeon HD 5000 series and later cards are supported. Those using older cards in the HD line will be stuck at 12.4b, baring any other other stopgap drivers provided by AMD.