Trojan is a malicious computer program which is used to hack into computers misleading users. Trojan or Trojan horse is a computer program that is malicious and is used to hack into any computer by misleading users of its true content. It always sits in system's background and perform harmful procedures. It is mostly disguised as e-mail attachements or by drive-by download. Trojans may allow the attacker to access user's personal informations like banking, passwords and personal identity.