When you use the search text box, only users that meet the search criteria in any of the four name fields display. For example, if you typed mark into the search box, any user with mark in the login, display, first, or last name display. This search is case insensitive.
Click on any of the A-Z letters to show only users whose login, display, first, or last name begin with that letter. To return to seeing all users, click on All.