The Silmarillion is a book written by J. R. R. Tolkien. It was first published posthumously (after Tolkien was dead) in 1977, edited by his son Christopher Tolkien. The Silmarillion is set in Tolkien's world Middle-earth, and is about its early history before The Hobbit and The Lord of the Rings.
The name "Silmarillion" can mean the published book (as in The Silmarillion), but it can also mean all the stories from the First Age of Middle-earth (the Quenta Silmarillion).