VoiceXML vs SALT

Hi,

I am relatively new to voice dialogs.
Could you please describe me - what are the differences between VoiceXML and SALT ?
As I understood for the moment both of them are used for the same purposes - programming of the dialogs.
If this is correct then it seems that having two standard families is redundant.

Please clarify the situation, if it is possible.

Thanks,
Ildar