I\'m not quite pro with encodings, but here\'s what I think I know (though it may be wrong):
First of all you'll find plenty of information in this SO topic.
ASCII is a charset, not encoding. Now, there's a number of 8-bit charsets, one of them being set as default in the system (you can change it in Regional Settings). *A functions accept 8-bit characters in that charset. UTF-8 is not a charset, but encoding of Unicode charset. *W functions, as I understand, use UTF-16 rather than UCS-2.