It looks like Microsoft are splitting out the Windows 10 search functionality from Cortana. Currently Microsoft combine Cortana and search into one UI launched from a combined search box in the task bar. With build 18298 on some systems, the search box is now just search with the Cortana functionality split into separate Cortana app. It looks like the split was introduced after the update, as during my hands on video on Tuesday Cortana was still part of search.
The search app shows recent activities as well as searching your system and the web. The Cortana app now has no direct typing option, it can only be controlled by voice. It seems like all the current functionality of the combined search is now in Search with just the Cortana Notebook and Devices settings in Cortana.
The combined Cortana search was unpopular with many Windows 10 users, as they just wanted search without the Cortana functionality. Split them up gives a simplified search experience and gives Microsoft room to bring new features to Cortana.
The new search box: