Fill rate: This indicates the speed at which the graphics card can "paint" a scene. Each surface in a 3D scene has to have a texture applied. The pixels (or texels) per second rating indicates how many pixels of textures can be painted per second. What is a texel? It is kind of like a 3D pixel. For instance, a flat surface may be painted (textured) with a 640 by 480 image of bricks, in order to make it look like a brick wall. Each pixel in that texture is known as a texel, and each has to be processed to take into account of the distance from and angle to the viewer of the wall. Other textures, such as light maps, can then overlay the original texture.
The fill rate is a product of the clock speed of the processor and the number of pixels it can process per clock cycle. The number of pixels that can be processed per clock cycle is gleaned by the number of pipelines the graphics chipset supports.
Vertices/triangles: Where the fill rate tells you the rendering performance of the graphics chip, it does not cover the performance of the card in geometry calculations. The graphics chips cannot work on curves - they can only process flat surfaces; but if you have enough flat surfaces you can make them look like curves. In a process called tessellation, all objects in a 3D scene are broken down into a set of triangular surfaces - the more triangles, the better the curves and angles of the real world can be represented. A 3D object can be made up of hundreds or even thousands of triangles. This is what game developers are talking about when they talk about the number of polygons in a character - the number of discreet (flat) surfaces in the character model.
Unfortunately, to further confuse matters, ATI and Nvidia talk about geometry processing power using different standards. Nvidia chip specifications refer to vertices per second (that is, the points at the corners of the triangles), while ATI specifications list triangles per second. (You cannot, incidentally, just divide the Nvidia numbers by three to get triangles - triangles next to each other share vertices).
Anti-aliasing: A technique used to make scenes look better by eliminating visual artefacts such as the jagged stepping effect caused by diagonal lines and square pixels. (Open up a paint program and draw a diagonal line, and you'll see what we mean). Anti-aliasing does improve image quality, but usually causes a performance hit. There are multiple levels of anti-aliasing, and some reduce performance more than others.
RAMDAC: This stands for Random Access Memory Digital to Analogue Converter. This is the chip that takes the scene and converts it to the format that your screen uses. The graphics processor creates a final image of a scene - the RAMDAC takes the image and puts it on the VGA or DVI cable for your monitor to display it. RAMDACs have different speeds - usually 350MHz or 400MHz, and a graphics card may have more than one. A faster RAMDAC indicates that the graphics card can support higher output resolutions, and multiple RAMDACs indicates that the card can support multiple displays, possibly in a dual-head configuration.