UPDATE #2: Apple released design guidelines for third-party Apple Watch bands on May 5, 2015. The Made for Apple Watch program is a lot like Apple's Made for iOS program which governs how manufactures create accessories for iOS devices. This means Apple-approved third-party bands will be available for the Apple Watch very soon.
There will be six Apple-created bands available for the Apple Watch at launch. The smartwatch will also be available in three different collections: Apple Watch, Apple Watch Sport and Apple Watch Edition, so you'll have a lot of different combinations to choose from.
UPDATE: Here's all the band styles that will be available for the Apple Watch:
Space Black Stainless Steel