MUO on MSN

I replaced my full Windows desktop environment with a lightweight one and it's insanely fast

Unlike traditional shell replacements like Cairo Shell or Open-Shell that are built with native Windows APIs and compiled C++ ...
Ars Technica

Windows Shell Replacements

OK, explain this to me... What exactly is a Windows Shell Replacement? I understand that it replaces the desktop and all that stuff. But what effects does it have on system performance? And is it safe ...