The Browser option includes a face browser, which is a nice way to personalize the login screen. A face browser allows users to enter their usernames by double-clicking on an icon, which usually is their picture. The face browser is not essential, as you can always just type your username in the prompt box.
GDM (the GNOME Display Manager) provides an alternate display manager for the X Window System.
The X Window System by default uses the XDM display manager. However, resolving XDM configuration issues typically involves editing a configuration file. GDM allows users to customize or troubleshoot settings without having to resort to a command line. Users can pick their session type on a per-login basis. GDM also features easy customisation with themes.

The GNOME Display Manager (GDM) is a display manager that implements all significant features required for managing attached and remote displays. GDM was written from scratch and does not contain any XDM or X Consortium code.
Note that GDM is configurable, and many configuration settings have an impact on security. Issues to be aware of are highlighted in this document. Please note that some Operating Systems configure GDM to behave differently than the default values as described in this document.
If GDM does not seem to behave as documented, then check to see if any related configuration may be different than described here. For further information about GDM, refer to the project website at http://wiki.gnome.org/Projects/GDM.

GDM 2.20 and earlier supported stable configuration interfaces. However, the codebase was completely rewritten for GDM 2.22, and is not completely backward compatible with older releases. This is in part because things work differently, so some options just don't make sense, in part because some options never made sense, and in part because some functionality has not been reimplemented yet. Interfaces which continue to be supported in a stable fashion include the Init, PreSession, PostSession, PostLogin, and Xsession scripts. Some daemon configuration options in the <etc>/gdm/custom.conf file continue to be supported.
In addition, the ~/.dmrc, and face browser image locations are still supported. GDM 2.20 and earlier supported the ability to manage multiple displays with separate graphics cards, such as used in terminal server environments, login in a window via a program like Xnest or Xephyr, the gdmsetup program, XML-based greeter themes, and the ability to run the XDMCP chooser from the login screen. These features were not added back during the 2.22 rewrite.