Aragorn is the central character in "Lord of the Rings". He does not appear in "The Silmarillion".

The Silmarillion is J.R.R. Tolkien's magnum opus. He began to work it in the trenches of Somme in 1916 during the World War One, and his son Christopher Tolkien edited and published it in 1977, four years after his death.

The Silmarillion is the history of the whole Tolkien's world, Arda (of which Middle-Earth is only a part) since its creation until the events of the Lord of the Rings. The basic set up is the battle between good and evil, the corruption, decay and destruction, and the hope which springs from the least of the creatures. While Men are the primary race in the Lord of the Rings, The Silmarillion is told from the Elven vantage point, and most of the major characters are Elves.

The Silmarillion is simultaneously an incredibly sad and a beautiful story, and while it is extremely tedious reading, it seldom leaves anyone cold.

More Info: en.wikipedia.org