What is Durarara about?

In Tokyo's downtown district of Ikebukuro, amidst many strange rumors and warnings of anonymous gangs and dangerous occupants, one urban legend stands out above the rest – the existence of a headless “Black Rider” who is said to be seen driving a jet-black motorcycle through the city streets.

Ryuugamine Mikado has always longed for the excitement of the city life, and an invitation from a childhood friend convinces him to move to Tokyo. Witnessing the Black Rider on his first day in the city, his wishes already seem to have been granted. But as supernatural events begin to occur, ordinary citizens like himself, along with Ikebukuro's most colorful inhabitants, are mixed up in the commotion breaking out in their city.
(I got this from animefreak.tv)

Mikado Ryūgamine is a young boy who longs for the exciting life of the big city. At the invitation of his childhood friend Masaomi Kida, he leaves home to go to a high school, Raira Academy, in Ikebukuro, Tokyo.

Masaomi warns him about people he doesn't want to cross in the city: a violent man dressed like a bartender, an information broker, and a mysterious gang called "Dollars." And, to top it off, Mikado witnesses an urban legend on his first day in the city: the "Black Rider," the supposedly headless pilot of a black motorcycle that sounds like a horse.

The narrative follows all of the characters equally, showing how their lives intersect, creating a greater plotline from what each character knows about a common incident