An important part of the X Window System user experience on modern Unix-like systems is the graphical login screen, in X11 parlance called a display manager. Most full desktop environments include one, and KDE is no exception, containing the prosaically-named kdm (for KDE Display Manager).
By default, kdm gives the user a simple dialog box with entry fields for the username and password. It is usually configured to provide a list of available users along the left-hand side of the dialog. Being a KDE application, it shares the look and feel of the rest of the KDE desktop, while still allowing the user to select other desktop environments (such as GNOME) or window managers (such as Window Maker) from a drop-down menu. Thus, the use of kdm as the login screen does not restrict all users of a system to using KDE, giving the choice characteristic of Linux and BSD systems.
KDM does not have to share the look and feel of its parent desktop, though; various themes are available that may mimic other operating systems or merely appear fancier than the 'serious' default look. This feature is more often associated with kdm's GNOME counterpart gdm, which defaults to a more free-form look than the standard login box, but is also present in kdm.
Not only does kdm handle system logins, it also handles system shutdowns and reboots on most desktop systems. (This feature is usually disabled on servers, as it is undesirable for ordinary users to shut them down.) In addition to providing these options from the kdm login screen, it also connects with the KDE logout command to provide shutdown and reboot options along with the usual 'just log out' option. KDM can also be configured to automatically log in a user upon system startup; this is considered insecure but is convenient.
Though kdm is usually used for a single terminal, it can be used on a server to handle connections from multiple terminals simultaneously. These terminals need only run an X server; the kdm login screen runs off the server, along with the ensuing desktop session and applications. Many Linux-based computer labs are built in this fashion, generally using software from the Linux Terminal Server Project; it is an economical setup since the terminal machines need not be especially powerful.
KDM is configurable through both a traditional Unix-style text file and an administrator-only component of the KDE Control Centre. In general, it is a user-friendly login manager with more powerful features accessible for specialized circumstances. It is included in the kdebase component of the KDE distribution, and is correspondingly included in many Linux distributions.
For BQ2K6. Copyright 2006 under the usual Creative Commons BY-SA-NC licence.