An illegal website do not have a license.It is not following the guideline of ethics and moral of the internet.It does no good.That is a fake website.
An illegal website is one that contains information that (document, videos etc) that has been considered to be illegal by your state/country/region. If you are found to be in possession of information obtained from these websites you can be prosecuted by law. These website may also contain virus and/or spyware which can be dangerous to your computer. You can also report these websites to the relevant authorities or security agencies so that they can be closed down.
An illegal website is a site that contains information (documentation/images/videos/etc) that has been deemed illegal by your state/nation for one reason or another. If you are found to be in possession of information obtained from one of these websites, you can be arrested, and even placed in prison for a long period of time.