The Window Manager is the layer between X windows and the desktop environment. Window managers (WM) control the placement & appearance of windows. The default in Puppy Slacko is JWM, its very light weight but effective. If you imagine a computer system as a series of layers it would look something like this:

Hardware < Operating System < Window System < Window Manager < Desktop Environment

Puppys default is JWM or (OpenBox in Lucid), see the What are JWM and OpenBox? thread.

Hardware < Linux < X Windows < JWM or OpenBox

(JWM or OpenBox also act as the Desktop Environment.)

JWM - default in Puppy4 and Slacko 5.3x, also see JWMElements.
OpenBox - default in Puppy 5.2x Lucid, not included in Puppy 5.3x Slacko.

Afterstep - Afterstep forum page
Antico - ANTICO forum page
Beryl - has special effects
Compiz - has special effects
Compiz-Fusion - has special effects
Dwm - Dwm forum page
Enlightenment - Used by MacPup
Fluxbox - Fluxbox forum page
Flwm - Flwm forum page
IceWM- Easy to use, customizable and good looking window manager.
Metisse - has special effects
PekWM
Project Looking Glass - has special effects
RatpoisonWM - minimalist with the emphasis on speed
XFWM - has special effects

JWM's elements
Window Manager Switcher
How to Change Window Manager
Window Managers with Special Effects
Desktop Environments

WikiPedia:Window_manager
List of window managers