1. Lock X11 display until password is entered. xlock locks the X server till the user enters his/her password at the keyboard. While xlock is running, all new server connections are refused. The mouse cursor is turned off. The screen is blanked and a changing pattern is put on the screen. If a key or a mouse button is pressed then the user is prompted for the password of the user who started xlock. This version is built without additional OpenGL/MesaGL and XPM modes. The companion package xlockmore-gl contains them. This is the only difference between the two packages. From Debian 3.0r0 APT

2. The xlockmore utility is an enhanced version of the standard xlockprogram, which allows you to lock an X session so that other users can't access it. Xlockmore runs a provided screensaver until you type in your password. Install the xlockmore package if you need a locking program to secure X sessions. From Mandrake 9.0 RPM