Twm (Tab Window Manager, or sometimes Tom's Window Manager, after the principal author Tom LaStrange) provides titlebars, shaped windows, several forms of icon management, user-defined macro functions, click-to-type and pointer-driven keyboard focus, and user-specified key and pointer button bindings.
It has however been left behind by more recent window managers, making it something of mainly historical interest. It was for some time the only real choice of window manager, after taking over from the very simplistic uwm. Nearly every window manager since has borrowed heavily from it, either directly, or indirectly through other window managers, such as fvwm.
This is probably the most basic version of Xorg you can get!! Works great on older hardware.