twm is a window manager for X11 built against Xlib.
Ranked in these QuestionsQuestion Ranking
Pro Was adopted as the default window manager for X11
twm was renamed Tab Window Manager (from Tom's Widow Manager) when it was adopted as X11's default window manager back in 1982. It was left behind many years ago but still stands as the foundation for every window manager that followed.
Pro Provided the base code for many other window managers
The code of twm was often built upon to create newer window managers such as FVWM.
Pro Simple and fast
Con Can be difficult to use
Most aspects of twm's interface operate differently than the more common UIs used in computing, thus reading the manual will more often than not be a prerequisite to using it.