99.99999999% of the population don't bother such specs. They only care about the price and only the price.
But for me who is quite a hardcore graphics card lover and likes to always explore every nook and crany about graphics cards, just let me indulge in explaining to you the differences.
Pixel Fillrate
The number of pixels the card can render to the screen every second. Before pixel shader processing became the more limiting factor, this was the most accurate measure of performance (along with texel fill rate).
Pixel rate is the maximum amount of pixels the GPU could possibly write to the local memory in one second, measured in millions of pixels per second. The actual pixel output rate also depends on quite a few other factors, most notably the memory bandwidth - the lower the memory bandwidth is, the lower the ability to get to the maximum fill rate.
This is calculated by multiplying the number of ROPs (Raster Operations Pipelines - aka Render Output Units) by the the core clock speed.
Render Output Units : The pixel pipelines take pixel and texel information and process it, via specific matrix and vector operations, into a final pixel or depth value. The ROPs perform the transactions between the relevant buffers in the local memory.
Importance : Higher the pixel rate, higher is the screen resolution of the GPU.
Texture fillrate
The number of textured pixels the card can render to the screen every second. To render a 3D scene, textures are mapped over the top of polygon meshes. This is called texture mapping and is accomplished by texture mapping units (TMUs) on the videocard. Texture fill rate is a measure of the speed with which a particular card can perform texture mapping.
Texel rate is the maximum number of texture map elements (texels) that can be processed per second. It is measured in millions of texels in one second
This is calculated by multiplying the total number of texture units by the core speed of the chip.
Texture Mapping Units : Textures need to be addressed and filtered. This job is done by TMUs that work in conjunction with pixel and vertex shader units. It is the TMU's job to apply texture operations to pixels.
Importance : Higher the texel rate, faster the game renders displays demanding games fluently.
Hopefully this will clarify your doubts.
