An autistic savant has both a severe developmental or mental handicap, and extraordinary mental abilities not found in most people. (Think Rainman.)
Most autistic savants have very extensive mental abilities, called splinter skills. They can memorize facts, numbers, license plates, maps, and extensive lists of sports and weather statistics. Some savants can mentally note and then recall perfectly a very long sequence of music, numbers, or speech.
Why autistic savants are capable of these astonishing feats is not quite clear. Some savants have obvious neurological abnormalities, but the brains of most savants are anatomically and physiologically normal; at least, there is no abnormality that modern science can detect.
There are only about 100 recognized prodigious savants in the world. Here is the story of Daniel Tammat:
And here is your bonus video with purchase! :-) Stephen Wiltshire can memorize and draw an entire cityscape: