Moments in Photos are the new Events, i.e. groupings of photos sorted by date taken.
When the iPhoto Library was first migrated to Photos there is a folder created in the sidebar titled iPhoto Events and all migrated iPhoto Events (which are now Moments) are represented by an album in that folder. To open the sidebar if it's not already open use the Option+Command+S key combination.
NOTE: It's been reported by several users that if the Event albums are moved out of the iPhoto Library folder in the sidebar they may disappear. It's not widespread but several users have reported that issue. Therefore, if you want to assure that you keep those Event albums don't move them outside the iPhoto Events folder.
There's a way to simulate events in Photos.
When new photos are imported into the Photos library go to the Last Import smart album, select all the photos and use the File ➙ New Album menu option or use the key combination Command+N. Name it as desired. It will appear just above the iPhoto Events folder where you can drag it into the iPhoto Events folder
When you click on the iPhoto Events folder you'll get a simulated iPhoto Events window.
The downside to the simulation is that the Albums/Events can only be sorted automatically by Title. But they can also be sorted manually, either in the sidebar or in the folder's window at the right. This will change with the release of Photos 2 with El Capitan this fall. That will allow the sorting by date or title, ascending or descending.
Tell Apple what missing features you'd like restored or new features added in Photos via Apple-Photos Feedback.
For more differences between iPhoto and Photos see this user tip: Photos 2.0 vs iPhoto 9.6.1: Features and Capabilities