double underscore means '__'

__ is the only global namespace used by this library.

why there is another library?

• all most all javascript library do things such as array utilities, string utilities, DOM, Event, Animation, Drag & Drop, widgets, browser compatibilities, layout management, standard components ... If we do NOT used a library, we had to cope with those thing ourselves, we lost our development efficiency. If we use a library, generally we had to load a big extra library file, we lost executive efficiency.
• double underscore tries to solves those problems. double underscore is NOT a all in one library, it only cope with things we are most probably faced with in daily web development. And it used a uncoupling structure which means you could always optimize your loading size by easily remove those unused parts.