Tiesto is the number one Trance Music producer and DJ in the world, as of this writting (Feb 2005). He became popular because of the popularity of trance music in the mid 90s and his remix of the song "Silence" by Delirium. He has the ability to reach out to the masses even to people who are not into trance. One great example of this is when he played at the opening cermony in the 2004 Athens Summer Olympics. No DJ/Producer had ever played at any opening ceremony in the Olympics, yet Tiesto was able to claim the crown and played in front of billions of people worldwide.
He is a very skilled producer but *sometimes* his productions are not as superb as one would expect but generally ther're excellent. Anyway, even though he is labeled as a trance music producer/dj sometimes his songs have more of a house feel to them compared to other songs produced by others. Anyway, generally his songs sound deep/dark but you'll find some happy ones too (Love Comes Again)