Antialiasing -> Simple explanation. This is to get rid of the "jaggies" on diagonal lines. Look at the picture below.

It boils down to this. To recover a signal, or image, you need a minimum of samples to be able to give a realistic representation of the image. The problems start with texture maps being either too close or too far away from the viewpoint. If the polygon is far away then you only have a limited number of points to show the texture map, so logically you have to drop a lot of the real pixels of your texture map.

Antialiased line - If you draw a straight line (under an angle) using a paint program and you zoom in, you will discover that the line looks like a stairway.  To remove this and make the line look like a line points in different colors are added to the side of the line to make it look more like a real line.

This creates some sort of interlace effect : one line is shown and one is not. This can result in weird patterns appearing, and makes the texture map look completely different from the real one. A similar problem is if the polygon is close to you. You need more info than there is available, resulting in the generation of random noise (meaningless values).

One of the best features compared to the competition is ATI's approach towards Antialiasing. In the name of performance ATI introduced Temporal (time) AA.

This is a feature that has been an option ever since the introduction of the R3xx processors, however it has never been used, this AA mode is to use different sample patterns. In simple wording, you sample the source at many consecutive points in time and then combine the samples for final output. This method basically is rendering the image at a much higher framerate than the final display, and sampling down to the fewer final rendered images. ATI's method could, perhaps, be better described as a multi-pass spatial AA method, jittered AA, or even temporal dithering. The advantage? Get this, 2x Temporal AA offers about the same quality as 4xAA while requiring only the performance of 2xAA.

The big downside of Temporal AA is that when framerates drop very low you'll notice an effect that is irritating, flickering. That means that most new games that demand a lot of the graphics core, or systems that are not powerful enough to cough up high framerates are not well suited for this AA technology at all. This will never be ATI's primary AA technology.

Unfortunately, due to the obligatory V-Sync, this AA mode cannot be benchmarked correctly.