The User Preferences screen (accessed with the link My Prefs on each page) presents two tabs for the user:

Preferences#

Name
This is your "Wiki" Name and is usually set to the Full Name (from the profile tab, below) without spaces
Editor
This is a preference setting for the editor to be used when you are creating or modifying wiki pages.
Section Editing
If toggled, a link will be placed beside each header, if you are allowed to edit that page.
Skin
This shows the different styles that the wiki may be shown to you. Skins are defined in the templates directory.
Language
This controls the language of the wiki - not the language that the pages are presented in.
Favorites Style
This controls which side of the screen the menu is presented on (default is Left side)
Time Format
This controls the format that dates are presented in page history screens
Time Zone
This is your time zone, if it is different from where the server is set.
Clear User Preferences
Pressing this button will clear the cookies used to remember your settings.

Profile Tab#

Login Name
This is the user name that is used in the login screen.
Password
Your password. It stored, hashed, using the SHA-1 hashing algorithm
Name
This is your Full Name, and is used by an administrator to provide membership in Groups or access control lists
Email Address
Your email address, in case you need your password reset
Roles
This shows the roles that you have, for security purposes (usually All and Authenticated)
Groups
This shows the groups that your user is a member of