From LXF Wiki

TWM is "Tom's Window Manager" or "Tab Window Manager", depending on who you ask. It is a window manager for X, and is as close to a 'standard' WM as there can be (it's usually included with X packages).

At first glance, TWM looks very bare; it's actually surprisingly usable though, and offers far more features than the truly minimalist WMs. TWM still looks dated, though, and most users have since moved on to derivatives such as CTWM.