And now normal users who already have apple's accounts (apple Id) can use the beta version of these new services: Pages,Numbers and Keynote. and for who don't have an acount he can just visit icloud.com and sign up to use these services.
Iwork for icloud has been in the beginning just for users who have developers account (not normal account), but apple has test the service on specific groupe of normal users who recieved an invitations to try the Iwork apps then apple decided to allow this service for everyone.
Also apple has announced for the first time about this service on the past Developers conference WWDC- june- 2013 and say that Iwork for icloud will be available for public on Beta version.