A compositor draws multiple windows on to the screen, taking care of the movements of windows. For example, it takes care of adding shadows, motion blur, animations, etc. It does this by first drawing windows onto a virtual screen, and then compositing them together on to the main screen.

Compositing window managers might have an easier time with synchronizing window states with the compositor.

Compositors can be divided based on the windowing system they use. It's either X Windowing System or the newer Wayland on UNIX-like operating systems, generally.

Examples

X Windowing System based

Standalone compositors

Picom (fork of compton)
Compton (dead fork of xcompmgr-dana, a fork of xcompmgr)
xcompmgr

Compositing window managers

Compiz (OpenGL based)
Metacity (XRender based)
Mutter (used by GNOME)
KWin (KDE Window Manager)
Enlightenment

Wayland based (compositing window managers)

Mutter (used by GNOME)
KWin (KDE Window Manager)
Enlightenment
Hyprland
Sway
dwl
River
Cage
Cagebreak
waymonad
wlroots (A base library for building Wayland compositors)
Weston (Wayland demo compositor)

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