The Necronomicon is a fictional medieval grimoire originating from the works of H.P. Lovecraft. In his mythos, the Necronomicon is believed to have been written by the Mad Arab Abdul Alhazred (not to be confused with the other mad Arab, Crazy Hassan). The book supposedly contains information on the Old Ones and magical spells to summon or ward them off, create zombies, and so on. Said information is so unnerving that simply reading the book can drive a person insane with Things Man Was Not Meant To Know. Notably, Lovecraft's stories mentioned several different translations, each of varying accuracy and scarcity. The book was one of several invented by Lovecraft, but as it was the most frequently used (some stories use it as a central plot device) it is that stuck his fanbase's mind. Both the sci-fi and fantasy genre often reference the Necronomicon, usually in a humorous way (Monsternomicon, Necronomicomicon, Necrotelecomnicon etc). The horror genre usually takes the book as it is, since Lovecraft's creations have become a sort of meme, shared by the collective.
Many believe the book to be real, despite Lovecraft's assertion to the contrary and several copies have surfaced, for example the Simon Necronomicon, which mixes the original Cthulhu Mythos with Sumerian mythology and medieval magical practices (like the Solomon Key). The title Necronomicon is also given to a huge leather-bound collection of all of Lovecraft's stories, so it's pretty easy to get the damn thing onto your bookshelves. There is also a collection of the art of HR Giger titled "Necronomicon" (and a "Necronomicon 2"!), which just figures.
The Necronomicon is a plot device in the Evil Dead movies. Characters use it to inadvertently summon the eponymous evil dead (AKA Deadites) to whatever isolated location they occupy. At the end of Evil Dead 2: Dead by Dawn, it sends Ash (Bruce Campbell) about a thousand years into the past and across the ocean to an early Medieval European realm for the events of Army of Darkness. Thus proving that even books of pure undiluted evil can still be pretty fucking awesome.
It was also cited repeatedly by Jack Chick as being an actual book and interconnected to Dungeons & Dragons. This is because Chick as a rule never had any idea what the fuck he was talking about.