Well... Internet sex is a very broad term and could mean many things, but in general it's some form of communication over the internet which involves sexual/erotic conversation. This can be in the form of: Text chat, Voice chat or even Video chat. The most common form of internet sex is Roleplay (cybering) where the individuals will pretend to be a particular scenario and imitate the actions through text chat e.g. *I unbutton my shirt* and this would stimulate arousal between the individuals. For voice chat, usually individuals would say would they would sexually do to each other and video chat, well... I don't think that needs much explanation. I hope that's given you some insight!
And to add a bit to what Pecnut here forgot:
Never EVER go looking for it on stranger chat sites. EVER! Nobody likes imbeciles like that. If you honestly have some sexual deficit or something and you HAVE to do it then just do it in private or if the other person wants it AFTER you've talked a bit.