When a person, usually male, on the internet changes their listed gender to the opposite one on some website. Some people only do this for attention, but there are others who feel seriously about it and want to begin living on the internet as the opposite sex since they can't do it in real life.
Did you hear about John's internet sex change? He changed his gender on Last FM to 'female' and has been acting like he has always been a woman. Maybe he has been, it's the internet so you can never be completely sure.