Cortana is part of the search interface, as demonstrated on video earlier this month. Apparently Cortana is only partially functional in this build, though interests, the notebook, and reminders can all be accessed.
The other major new addition in build 9901 is the Xbox app. The app lets users view the Xbox Store, their achievements, friends list, and activity feeds. It's currently available only a sneak peak.
There are other, smaller changes in this build. There are new animations in the TaskView switcher, along with a new view for the settings app. The Windows Store has been updated as well, adding music, movies, and TV shows next to apps.
Source: The Verge