You can rate photos on a scale from ★ to ★★★★★.

To rate the selected photos, click one of the stars in the rating part of the Info window.

Or, type a number from 0 through 5 in the "add a cateogry" box in the Info window.

Or, click on a photo in the results and just type a number from 0 through 5. To rate lots of photos quickly, you can choose to automatically go to the next photo when rating photos using the keyboard in the Photos preferences.

The rating of each picture is displayed in both thumbnail and list views.