Online identity theft is the practice of pretending to be someone else on the internet. The purpose can be quite harmless (like chatting with someone under someone else's account), but when referred to in the media, it's often about the criminal activity of stealing someone's personal information for his or her own financial gain. More often than not, it involves phishing for a person's banking information and using that to order goods or transfer money to another bank account.
From : en.wikipedia.org/wiki/Online_identity_theft