The graphics processing unit that the card is based on.
Graphics chip manufacturers produce a relatively small number of chips per series/generation. They use these few chips to come up with several reference cards.
Mostly chips are marketed on different boards by changing the clock speeds and/or coupling the chip with a different amount of memory or memory speed. Performance of two cards based on the same chip can vary widely due to clock speed changes.
Less frequently (because it is not cost-effective), manufacturers create lower performance versions of a given card by disabling part of the chip either in the driver, the bios, or by manipulating the chip itself with laser etching or some other method. In some cases, the process used to disable the pipelines can be reversed though a driver hack or a bios flash, thus increasing the performance of the chip.