Hi Nicola,
char and zstring are both null-terminated strings with 8bit characters. wchar and wzstring are null-terminated strings with 16bit characters. They are the same as the char types used in Windows.
bstring and wbstring are likewise windows OLE/BSTR strings.
string and wstring are oxygen-native strings based on the above. But they have automatic garbage collection.
You can use strings as char* parameters in Windows API calls. The compiler will sort it