[Haskell-beginners] When to use ByteString rather than [Char] ... ?

Hi, After working through a few Haskell tutorials, I've come across numerous recommendations to use the Data.ByteString library rather than standard [Char], for reasons of "performance". I'm having trouble swallowing this -- presumably the standard String is default for good reasons. Nothing has answered this question: in what case is it better to use [Char]? Could anyone point me to a good resource showing the differences between how [Char] and ByteString are implemented, and giving good a heuristic for me to decide which is better in any one case? Best, James Fisher