Morgoth is a fictional antagonist in the works of J.R.R. Tolkien. Appearing throughout a good portion of The Silmarillian, Morgoth is most easily analgised to the Christian concept of Satan, a powerful fallen angel who's agenda is to corrupt the earth.

There are a number of good character portraits on the web about Morgoth (who is also known as Melkor). The exploits of the character are a key part to understanding the early parts of Tolkien's mythology. The events of the Lord of the Rings are as far removed from the days of Morgoth, the destruction of the Two Trees, the creation of the Silmarils, etc. as modern western civilization would be from anceint Assyrian events. Yet in Tolkien's world there is a continuity that ties even the final battles of the War of the Ring to the waking of the Elves and before.
 

The main points of contention and strife caused by Morgoth always, ultimately, revolve around light. Before the Elves, Morgoth destroyed the Great Lamps, Later he caused the death of the Two Trees and stole the only remaining remnants of their light, which were contained in three great jewels; the Silmarils. These he put into his crown of iron, and so is often deplicted as having a great helm with three glorious lights shining from it. The desire to control or destroy all light on Ea seems to drive Morgoth.
 

Ultimately, Morgoth is a product of grander design of the creator, as are all things in the world. For all of his power and will to injure the "original" design, that too is a part of the design and without it there would be no need to tell the stories of Middle Earth. Ea without evil would have been a gorgous, boring place. The Silmarils would have still been beautiful, but seen in the light of the Two Trees, they would have been pale. Even the Two Trees wouldn't have been needed if the Great Lamps remained to light the world. Very boring indeed.