Audio editing is the manipulation of  length or specific timing of  single or multiple audio tracks within a song.  Originally done as tape splicing, the physical cutting and splicing of audio tape to improve a song or performance, goes back to the 1940's and the invention of the tape recorder itself.  All music has been spliced together since then (if not the manipulate a song, then to sequence a set of songs for the recording master).  Current technologies allow for much more intricate editing which can be tested in the computer environment with the ability to 'undo'  and retry limitless options.  Understanding music composition, traditional tape splicing techniques, along with in depth knowledge of computer audio software are important in great seamless musical editing.  Video editing follows a similar history, originally cutting physical film stock, which is now performed in computers (for the most part).