Ubuntu One is the single account you use to log in to all services and sites related to Ubuntu. Previously, it was also a cross-platform synchronization service for files, contacts, notes, music and bookmarks, now discontinued.
Ubuntu One is now the single account you use to log in to all services and sites related to Ubuntu. Your Ubuntu One account is an email address and a password that you use to sign in to Ubuntu services.
Ubuntu One was also Canonical's file and application synchronization service. As well as file sync, U1 allowed access to contacts, notes or bookmarks from other Ubuntu computers or through its web interface. It also included the Ubuntu One Music Store and could stream purchased music to iPhone and Android devices. This service was discontinued on 31 July 2014.