Sometimes the same song is done by more than one artist. Often time the original performer a song will have influenced or inspired another performer. This can lead to this other artist then doing their own version of that song. This can be done several ways, but the best covers show the style and substance of both the original artist and the person or group making the cover. It can also go completely wrong, and have the two version of the song sound almost identical.
Here is an example of good cover work.
Above are the original Queen version of "Radio Gaga" and the Electric 6 cover. While they are the same song, there is a huge difference in how they are performed. This shows how much a song can change with artists, as they aren't even the same genre of music.
On the other hand, here is a poorly done cover:
Here we see a cover that is nearly identical to the original. The singer on the cover is even going so far as to make is voice sound like the original artist. When doing a cover, an artist should try to make the song their own. What is the point of covering a song just to have be so close to the original. This only a critique of the cover, not of the song itself.