The Dark Web is a part of the internet that cannot be accsessed using a normal browser such as Chrome, Firefox, Internet Explorer etc.
The Dark Web is mainly used for illegal activities, such as markets for drugs, child pornography chat rooms, hiring sites for hitmen and the like.
People access the Dark Web using the TOR browser.
The Dark Web is a part of the Deep Web as well. However, the Deep Web is not as malignant as the Dark Web.
I hope this helped!