A WM is literally just the window manager. It makes windows work and have a border and maximize and stuff but that’s it.
A DE gives you a lot of other stuff: a root window that makes up the “desktop”, panels & widgets, notification area, an application menu, workspaces, window and workspace switchers, global hotkeys, the concept of a session and stuff related to it (things to run on start, or saving your session between reboots), a unified theme and fonts etc. etc.
There are also programs that fall somewhere between these. For example tiling WM tend to fill the whole screen so they don’t care a lot about all the things I mentioned but they can integrate with some other stuff to some extent. Or something like OpenBox which includes a very lightweight desktop, menu and panel so I guess you could call it a DE but it’s all contained in one executable.