|It's a 56K dial-up modem that can also send and receive faxes - a 56K dial-up modem is much slower than a high speed modem - if you don't need to fax you probably don't need this card for that.|
ISDN is a type of high speed telephone line connection most people don't have access to (businesses might).
GSM is a cell phone feature?
There doesn't appear to be a manual for it
but there is some support info here