iOS 10's overhauled Photos app now identifies faces in your photos. Once you put a name to the face you will have an entry in your People album for pictures of that person, and you will be able to search for photos of them using their name. To give a person a name, tap on their photo, then tap on the "Details" button in the upper right corner. Scroll down until you see the "People" section, then tap on the photo again. Now you will see an option to "+ Add Name" in the top center. Tap on that and type in the name.
How do I identify / tag a person in Photos?