The clock speed of the graphics processor on the card.
Video cards have two clock speeds of interest, the memory clock, and the core clock. The core clock is the speed at which the graphics processor on the card operatores. The clock speed of a chip, combined with the number/configuration of the pipelines in the chip, give a pretty accurate picture of what the performance of the chip will be.
In many cases, the difference between two video cards of the same generation from the same company will differ only in core and memory clock speeds. The ability to set these clock speeds to whatever they want gives the manufacturer the ability to market multiple video cards from the same chip.
You'll notice that retail card clock speeds often differ from the reference card specification (especially with nVidia cards). This is because retail card manufacturers are given some room to adjust clock speeds in order to diffferentiate their card from every other card on the market based on the same reference design.
You can also manually change the core clock speed on most video cards using third party utilities, or, in some cases, the driver provided by the manufacturer. Most cards don't gain much performance from overclocking, but sometimes cards are released with a tremendous amount of over-clocking headroom and serious performance gains can be had simply by cranking up a dial in your driver control panel.
Typically, faster clocked cards will use more power, and thus produce more heat.
Also, note that some cards clock different parts of the chip at different speeds, and that most chips run at a slower clock frequency when in 2D mode (regular programs in windows, not 3D games).