J.R.R. Tolkien>
The Silmarillion
The Ainulindalë was the music that brought Arda (The World) into being. Ilúvatar gave to each of the Ainur special powers and a limited knowledge of the Music which pertained to their powers. Thus, they together formed the orchestra that formed the world, though they did not know what they were creating until Ilúvatar spoke 'Eä!' (Be!), and showed them Arda.
There were three main themes to the Ainulindalë, which chronicled the three ages of the world. (Not to be confused with the First Age-Fourth Age as portrayed in the Silmarillion.) The Ainu who Ilúvatar had given the greatest power and knowledge in all things was Melkor. He was impatient with the designs of his creator and desired to bring things of his own mind into Being. Thus, he created discord in the Music and marred it and its creation. In this he caused some of the Ainu to try to harmonize with his melody and others were silenced completely.
The Ainur only participated in two of the three themes. The first which Melkor marred, the second in which Ilúvatar abated Melkor's discord, and the third which Ilúvatar alone made. In this third theme was the Children of Ilúvatar (Elves and Men) and their ultimate fate dealt with. This is why the Valar have such wonder for the Children, they have no knowledge of their creation or destiny.
In my own
observations, I have realized what an amazing
parallel the Ainulindalë has with the current
Superstring theory. Both are basically stating that the
Universe is made up of a sort of
cosmic music that is being played out like a grand
symphony. Though I'm sure
Proffesor Tolkien had no
inkling (pun intended) of current
physics, it's amazing how similar they both are (At least in a broad view of them).