The user list, accessed via the 'Users' link at the top of each page, is basically a listing of all the users that have registered on the site. 
 
If you are looking to search for a particular user by all/part of their username, then in the drop-down box at the bottom of the page, change the selection from 'Search All Available' to 'Name Begins With' or 'Name Contains' and input all/part of their name in the text input field and press the 'Go!' button. 
 
Also, at the bottom of the user list page, there are a number of sorting options available to alter the way in which the list is displayed. 
 
If a user has chosen to add a photo to their profile information, then a camera icon will appear next to their name, and you may click this to view the photo.